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+# sql/schema.py
+# Copyright (C) 2005-2022 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: https://www.opensource.org/licenses/mit-license.php
+
+"""The schema module provides the building blocks for database metadata.
+
+Each element within this module describes a database entity which can be
+created and dropped, or is otherwise part of such an entity.  Examples include
+tables, columns, sequences, and indexes.
+
+All entities are subclasses of :class:`~sqlalchemy.schema.SchemaItem`, and as
+defined in this module they are intended to be agnostic of any vendor-specific
+constructs.
+
+A collection of entities are grouped into a unit called
+:class:`~sqlalchemy.schema.MetaData`. MetaData serves as a logical grouping of
+schema elements, and can also be associated with an actual database connection
+such that operations involving the contained elements can contact the database
+as needed.
+
+Two of the elements here also build upon their "syntactic" counterparts, which
+are defined in :class:`~sqlalchemy.sql.expression.`, specifically
+:class:`~sqlalchemy.schema.Table` and :class:`~sqlalchemy.schema.Column`.
+Since these objects are part of the SQL expression language, they are usable
+as components in SQL expressions.
+
+"""
+from __future__ import absolute_import
+
+import collections
+
+import sqlalchemy
+from . import coercions
+from . import ddl
+from . import roles
+from . import type_api
+from . import visitors
+from .base import _bind_or_error
+from .base import DedupeColumnCollection
+from .base import DialectKWArgs
+from .base import Executable
+from .base import SchemaEventTarget
+from .coercions import _document_text_coercion
+from .elements import ClauseElement
+from .elements import ColumnClause
+from .elements import ColumnElement
+from .elements import quoted_name
+from .elements import TextClause
+from .selectable import TableClause
+from .type_api import to_instance
+from .visitors import InternalTraversal
+from .. import event
+from .. import exc
+from .. import inspection
+from .. import util
+
+
+RETAIN_SCHEMA = util.symbol(
+    "retain_schema"
+    """Symbol indicating that a :class:`_schema.Table`, :class:`.Sequence`
+    or in some cases a :class:`_schema.ForeignKey` object, in situations
+    where the object is being copied for a :meth:`.Table.to_metadata`
+    operation, should retain the schema name that it already has.
+
+    """
+)
+
+BLANK_SCHEMA = util.symbol(
+    "blank_schema",
+    """Symbol indicating that a :class:`_schema.Table`, :class:`.Sequence`
+    or in some cases a :class:`_schema.ForeignKey` object
+    should have 'None' for its schema, even if the parent
+    :class:`_schema.MetaData` has specified a schema.
+
+    .. versionadded:: 1.0.14
+
+    """,
+)
+
+NULL_UNSPECIFIED = util.symbol(
+    "NULL_UNSPECIFIED",
+    """Symbol indicating the "nullable" keyword was not passed to a Column.
+
+    Normally we would expect None to be acceptable for this but some backends
+    such as that of SQL Server place special signficance on a "nullability"
+    value of None.
+
+    """,
+)
+
+
+def _get_table_key(name, schema):
+    if schema is None:
+        return name
+    else:
+        return schema + "." + name
+
+
+# this should really be in sql/util.py but we'd have to
+# break an import cycle
+def _copy_expression(expression, source_table, target_table):
+    if source_table is None or target_table is None:
+        return expression
+
+    def replace(col):
+        if (
+            isinstance(col, Column)
+            and col.table is source_table
+            and col.key in source_table.c
+        ):
+            return target_table.c[col.key]
+        else:
+            return None
+
+    return visitors.replacement_traverse(expression, {}, replace)
+
+
+@inspection._self_inspects
+class SchemaItem(SchemaEventTarget, visitors.Visitable):
+    """Base class for items that define a database schema."""
+
+    __visit_name__ = "schema_item"
+
+    create_drop_stringify_dialect = "default"
+
+    def _init_items(self, *args, **kw):
+        """Initialize the list of child items for this SchemaItem."""
+        for item in args:
+            if item is not None:
+                try:
+                    spwd = item._set_parent_with_dispatch
+                except AttributeError as err:
+                    util.raise_(
+                        exc.ArgumentError(
+                            "'SchemaItem' object, such as a 'Column' or a "
+                            "'Constraint' expected, got %r" % item
+                        ),
+                        replace_context=err,
+                    )
+                else:
+                    spwd(self, **kw)
+
+    def __repr__(self):
+        return util.generic_repr(self, omit_kwarg=["info"])
+
+    @util.memoized_property
+    def info(self):
+        """Info dictionary associated with the object, allowing user-defined
+        data to be associated with this :class:`.SchemaItem`.
+
+        The dictionary is automatically generated when first accessed.
+        It can also be specified in the constructor of some objects,
+        such as :class:`_schema.Table` and :class:`_schema.Column`.
+
+        """
+        return {}
+
+    def _schema_item_copy(self, schema_item):
+        if "info" in self.__dict__:
+            schema_item.info = self.info.copy()
+        schema_item.dispatch._update(self.dispatch)
+        return schema_item
+
+    _use_schema_map = True
+
+
+class Table(DialectKWArgs, SchemaItem, TableClause):
+    r"""Represent a table in a database.
+
+    e.g.::
+
+        mytable = Table(
+            "mytable", metadata,
+            Column('mytable_id', Integer, primary_key=True),
+            Column('value', String(50))
+        )
+
+    The :class:`_schema.Table`
+    object constructs a unique instance of itself based
+    on its name and optional schema name within the given
+    :class:`_schema.MetaData` object. Calling the :class:`_schema.Table`
+    constructor with the same name and same :class:`_schema.MetaData` argument
+    a second time will return the *same* :class:`_schema.Table`
+    object - in this way
+    the :class:`_schema.Table` constructor acts as a registry function.
+
+    .. seealso::
+
+        :ref:`metadata_describing` - Introduction to database metadata
+
+    Constructor arguments are as follows:
+
+    :param name: The name of this table as represented in the database.
+
+        The table name, along with the value of the ``schema`` parameter,
+        forms a key which uniquely identifies this :class:`_schema.Table`
+        within
+        the owning :class:`_schema.MetaData` collection.
+        Additional calls to :class:`_schema.Table` with the same name,
+        metadata,
+        and schema name will return the same :class:`_schema.Table` object.
+
+        Names which contain no upper case characters
+        will be treated as case insensitive names, and will not be quoted
+        unless they are a reserved word or contain special characters.
+        A name with any number of upper case characters is considered
+        to be case sensitive, and will be sent as quoted.
+
+        To enable unconditional quoting for the table name, specify the flag
+        ``quote=True`` to the constructor, or use the :class:`.quoted_name`
+        construct to specify the name.
+
+    :param metadata: a :class:`_schema.MetaData`
+        object which will contain this
+        table.  The metadata is used as a point of association of this table
+        with other tables which are referenced via foreign key.  It also
+        may be used to associate this table with a particular
+        :class:`.Connectable`.
+
+    :param \*args: Additional positional arguments are used primarily
+        to add the list of :class:`_schema.Column`
+        objects contained within this
+        table. Similar to the style of a CREATE TABLE statement, other
+        :class:`.SchemaItem` constructs may be added here, including
+        :class:`.PrimaryKeyConstraint`, and
+        :class:`_schema.ForeignKeyConstraint`.
+
+    :param autoload: Defaults to ``False``, unless
+        :paramref:`_schema.Table.autoload_with`
+        is set in which case it defaults to ``True``;
+        :class:`_schema.Column` objects
+        for this table should be reflected from the database, possibly
+        augmenting objects that were explicitly specified.
+        :class:`_schema.Column` and other objects explicitly set on the
+        table will replace corresponding reflected objects.
+
+        .. deprecated:: 1.4
+
+            The autoload parameter is deprecated and will be removed in
+            version 2.0.  Please use the
+            :paramref:`_schema.Table.autoload_with` parameter, passing an
+            engine or connection.
+
+        .. seealso::
+
+            :ref:`metadata_reflection_toplevel`
+
+    :param autoload_replace: Defaults to ``True``; when using
+        :paramref:`_schema.Table.autoload`
+        in conjunction with :paramref:`_schema.Table.extend_existing`,
+        indicates
+        that :class:`_schema.Column` objects present in the already-existing
+        :class:`_schema.Table`
+        object should be replaced with columns of the same
+        name retrieved from the autoload process.   When ``False``, columns
+        already present under existing names will be omitted from the
+        reflection process.
+
+        Note that this setting does not impact :class:`_schema.Column` objects
+        specified programmatically within the call to :class:`_schema.Table`
+        that
+        also is autoloading; those :class:`_schema.Column` objects will always
+        replace existing columns of the same name when
+        :paramref:`_schema.Table.extend_existing` is ``True``.
+
+        .. seealso::
+
+            :paramref:`_schema.Table.autoload`
+
+            :paramref:`_schema.Table.extend_existing`
+
+    :param autoload_with: An :class:`_engine.Engine` or
+        :class:`_engine.Connection` object,
+        or a :class:`_reflection.Inspector` object as returned by
+        :func:`_sa.inspect`
+        against one, with which this :class:`_schema.Table`
+        object will be reflected.
+        When set to a non-None value, the autoload process will take place
+        for this table against the given engine or connection.
+
+    :param extend_existing: When ``True``, indicates that if this
+        :class:`_schema.Table` is already present in the given
+        :class:`_schema.MetaData`,
+        apply further arguments within the constructor to the existing
+        :class:`_schema.Table`.
+
+        If :paramref:`_schema.Table.extend_existing` or
+        :paramref:`_schema.Table.keep_existing` are not set,
+        and the given name
+        of the new :class:`_schema.Table` refers to a :class:`_schema.Table`
+        that is
+        already present in the target :class:`_schema.MetaData` collection,
+        and
+        this :class:`_schema.Table`
+        specifies additional columns or other constructs
+        or flags that modify the table's state, an
+        error is raised.  The purpose of these two mutually-exclusive flags
+        is to specify what action should be taken when a
+        :class:`_schema.Table`
+        is specified that matches an existing :class:`_schema.Table`,
+        yet specifies
+        additional constructs.
+
+        :paramref:`_schema.Table.extend_existing`
+        will also work in conjunction
+        with :paramref:`_schema.Table.autoload` to run a new reflection
+        operation against the database, even if a :class:`_schema.Table`
+        of the same name is already present in the target
+        :class:`_schema.MetaData`; newly reflected :class:`_schema.Column`
+        objects
+        and other options will be added into the state of the
+        :class:`_schema.Table`, potentially overwriting existing columns
+        and options of the same name.
+
+        As is always the case with :paramref:`_schema.Table.autoload`,
+        :class:`_schema.Column` objects can be specified in the same
+        :class:`_schema.Table`
+        constructor, which will take precedence.  Below, the existing
+        table ``mytable`` will be augmented with :class:`_schema.Column`
+        objects
+        both reflected from the database, as well as the given
+        :class:`_schema.Column`
+        named "y"::
+
+            Table("mytable", metadata,
+                        Column('y', Integer),
+                        extend_existing=True,
+                        autoload_with=engine
+                    )
+
+        .. seealso::
+
+            :paramref:`_schema.Table.autoload`
+
+            :paramref:`_schema.Table.autoload_replace`
+
+            :paramref:`_schema.Table.keep_existing`
+
+
+    :param implicit_returning: True by default - indicates that
+        RETURNING can be used by default to fetch newly inserted primary key
+        values, for backends which support this.  Note that
+        :func:`_sa.create_engine` also provides an ``implicit_returning``
+        flag.
+
+    :param include_columns: A list of strings indicating a subset of
+        columns to be loaded via the ``autoload`` operation; table columns who
+        aren't present in this list will not be represented on the resulting
+        ``Table`` object. Defaults to ``None`` which indicates all columns
+        should be reflected.
+
+    :param resolve_fks: Whether or not to reflect :class:`_schema.Table`
+        objects
+        related to this one via :class:`_schema.ForeignKey` objects, when
+        :paramref:`_schema.Table.autoload` or
+        :paramref:`_schema.Table.autoload_with` is
+        specified.   Defaults to True.  Set to False to disable reflection of
+        related tables as :class:`_schema.ForeignKey`
+        objects are encountered; may be
+        used either to save on SQL calls or to avoid issues with related tables
+        that can't be accessed. Note that if a related table is already present
+        in the :class:`_schema.MetaData` collection, or becomes present later,
+        a
+        :class:`_schema.ForeignKey` object associated with this
+        :class:`_schema.Table` will
+        resolve to that table normally.
+
+        .. versionadded:: 1.3
+
+        .. seealso::
+
+            :paramref:`.MetaData.reflect.resolve_fks`
+
+
+    :param info: Optional data dictionary which will be populated into the
+        :attr:`.SchemaItem.info` attribute of this object.
+
+    :param keep_existing: When ``True``, indicates that if this Table
+        is already present in the given :class:`_schema.MetaData`, ignore
+        further arguments within the constructor to the existing
+        :class:`_schema.Table`, and return the :class:`_schema.Table`
+        object as
+        originally created. This is to allow a function that wishes
+        to define a new :class:`_schema.Table` on first call, but on
+        subsequent calls will return the same :class:`_schema.Table`,
+        without any of the declarations (particularly constraints)
+        being applied a second time.
+
+        If :paramref:`_schema.Table.extend_existing` or
+        :paramref:`_schema.Table.keep_existing` are not set,
+        and the given name
+        of the new :class:`_schema.Table` refers to a :class:`_schema.Table`
+        that is
+        already present in the target :class:`_schema.MetaData` collection,
+        and
+        this :class:`_schema.Table`
+        specifies additional columns or other constructs
+        or flags that modify the table's state, an
+        error is raised.  The purpose of these two mutually-exclusive flags
+        is to specify what action should be taken when a
+        :class:`_schema.Table`
+        is specified that matches an existing :class:`_schema.Table`,
+        yet specifies
+        additional constructs.
+
+        .. seealso::
+
+            :paramref:`_schema.Table.extend_existing`
+
+    :param listeners: A list of tuples of the form ``(<eventname>, <fn>)``
+        which will be passed to :func:`.event.listen` upon construction.
+        This alternate hook to :func:`.event.listen` allows the establishment
+        of a listener function specific to this :class:`_schema.Table` before
+        the "autoload" process begins.  Historically this has been intended
+        for use with the :meth:`.DDLEvents.column_reflect` event, however
+        note that this event hook may now be associated with the
+        :class:`_schema.MetaData` object directly::
+
+            def listen_for_reflect(table, column_info):
+                "handle the column reflection event"
+                # ...
+
+            t = Table(
+                'sometable',
+                autoload_with=engine,
+                listeners=[
+                    ('column_reflect', listen_for_reflect)
+                ])
+
+        .. seealso::
+
+            :meth:`_events.DDLEvents.column_reflect`
+
+    :param must_exist: When ``True``, indicates that this Table must already
+        be present in the given :class:`_schema.MetaData` collection, else
+        an exception is raised.
+
+    :param prefixes:
+        A list of strings to insert after CREATE in the CREATE TABLE
+        statement.  They will be separated by spaces.
+
+    :param quote: Force quoting of this table's name on or off, corresponding
+        to ``True`` or ``False``.  When left at its default of ``None``,
+        the column identifier will be quoted according to whether the name is
+        case sensitive (identifiers with at least one upper case character are
+        treated as case sensitive), or if it's a reserved word.  This flag
+        is only needed to force quoting of a reserved word which is not known
+        by the SQLAlchemy dialect.
+
+        .. note:: setting this flag to ``False`` will not provide
+           case-insensitive behavior for table reflection; table reflection
+           will always search for a mixed-case name in a case sensitive
+           fashion.  Case insensitive names are specified in SQLAlchemy only
+           by stating the name with all lower case characters.
+
+    :param quote_schema: same as 'quote' but applies to the schema identifier.
+
+    :param schema: The schema name for this table, which is required if
+        the table resides in a schema other than the default selected schema
+        for the engine's database connection.  Defaults to ``None``.
+
+        If the owning :class:`_schema.MetaData` of this :class:`_schema.Table`
+        specifies its
+        own :paramref:`_schema.MetaData.schema` parameter,
+        then that schema name will
+        be applied to this :class:`_schema.Table`
+        if the schema parameter here is set
+        to ``None``.  To set a blank schema name on a :class:`_schema.Table`
+        that
+        would otherwise use the schema set on the owning
+        :class:`_schema.MetaData`,
+        specify the special symbol :attr:`.BLANK_SCHEMA`.
+
+        .. versionadded:: 1.0.14  Added the :attr:`.BLANK_SCHEMA` symbol to
+           allow a :class:`_schema.Table`
+           to have a blank schema name even when the
+           parent :class:`_schema.MetaData` specifies
+           :paramref:`_schema.MetaData.schema`.
+
+        The quoting rules for the schema name are the same as those for the
+        ``name`` parameter, in that quoting is applied for reserved words or
+        case-sensitive names; to enable unconditional quoting for the schema
+        name, specify the flag ``quote_schema=True`` to the constructor, or use
+        the :class:`.quoted_name` construct to specify the name.
+
+    :param comment: Optional string that will render an SQL comment on table
+         creation.
+
+         .. versionadded:: 1.2 Added the :paramref:`_schema.Table.comment`
+            parameter
+            to :class:`_schema.Table`.
+
+    :param \**kw: Additional keyword arguments not mentioned above are
+        dialect specific, and passed in the form ``<dialectname>_<argname>``.
+        See the documentation regarding an individual dialect at
+        :ref:`dialect_toplevel` for detail on documented arguments.
+
+    """
+
+    __visit_name__ = "table"
+
+    constraints = None
+    """A collection of all :class:`_schema.Constraint` objects associated with
+      this :class:`_schema.Table`.
+
+      Includes :class:`_schema.PrimaryKeyConstraint`,
+      :class:`_schema.ForeignKeyConstraint`, :class:`_schema.UniqueConstraint`,
+      :class:`_schema.CheckConstraint`.  A separate collection
+      :attr:`_schema.Table.foreign_key_constraints` refers to the collection
+      of all :class:`_schema.ForeignKeyConstraint` objects, and the
+      :attr:`_schema.Table.primary_key` attribute refers to the single
+      :class:`_schema.PrimaryKeyConstraint` associated with the
+      :class:`_schema.Table`.
+
+      .. seealso::
+
+            :attr:`_schema.Table.constraints`
+
+            :attr:`_schema.Table.primary_key`
+
+            :attr:`_schema.Table.foreign_key_constraints`
+
+            :attr:`_schema.Table.indexes`
+
+            :class:`_reflection.Inspector`
+
+
+    """
+
+    indexes = None
+    """A collection of all :class:`_schema.Index` objects associated with this
+      :class:`_schema.Table`.
+
+      .. seealso::
+
+            :meth:`_reflection.Inspector.get_indexes`
+
+    """
+
+    _traverse_internals = TableClause._traverse_internals + [
+        ("schema", InternalTraversal.dp_string)
+    ]
+
+    def _gen_cache_key(self, anon_map, bindparams):
+        if self._annotations:
+            return (self,) + self._annotations_cache_key
+        else:
+            return (self,)
+
+    @util.deprecated_params(
+        mustexist=(
+            "1.4",
+            "Deprecated alias of :paramref:`_schema.Table.must_exist`",
+        ),
+        autoload=(
+            "2.0",
+            "The autoload parameter is deprecated and will be removed in "
+            "version 2.0.  Please use the "
+            "autoload_with parameter, passing an engine or connection.",
+        ),
+    )
+    def __new__(cls, *args, **kw):
+        if not args and not kw:
+            # python3k pickle seems to call this
+            return object.__new__(cls)
+
+        try:
+            name, metadata, args = args[0], args[1], args[2:]
+        except IndexError:
+            raise TypeError(
+                "Table() takes at least two positional-only "
+                "arguments 'name' and 'metadata'"
+            )
+
+        schema = kw.get("schema", None)
+        if schema is None:
+            schema = metadata.schema
+        elif schema is BLANK_SCHEMA:
+            schema = None
+        keep_existing = kw.get("keep_existing", False)
+        extend_existing = kw.get("extend_existing", False)
+
+        if keep_existing and extend_existing:
+            msg = "keep_existing and extend_existing are mutually exclusive."
+            raise exc.ArgumentError(msg)
+
+        must_exist = kw.pop("must_exist", kw.pop("mustexist", False))
+        key = _get_table_key(name, schema)
+        if key in metadata.tables:
+            if not keep_existing and not extend_existing and bool(args):
+                raise exc.InvalidRequestError(
+                    "Table '%s' is already defined for this MetaData "
+                    "instance.  Specify 'extend_existing=True' "
+                    "to redefine "
+                    "options and columns on an "
+                    "existing Table object." % key
+                )
+            table = metadata.tables[key]
+            if extend_existing:
+                table._init_existing(*args, **kw)
+            return table
+        else:
+            if must_exist:
+                raise exc.InvalidRequestError("Table '%s' not defined" % (key))
+            table = object.__new__(cls)
+            table.dispatch.before_parent_attach(table, metadata)
+            metadata._add_table(name, schema, table)
+            try:
+                table._init(name, metadata, *args, **kw)
+                table.dispatch.after_parent_attach(table, metadata)
+                return table
+            except Exception:
+                with util.safe_reraise():
+                    metadata._remove_table(name, schema)
+
+    def __init__(self, *args, **kw):
+        """Constructor for :class:`_schema.Table`.
+
+        This method is a no-op.   See the top-level
+        documentation for :class:`_schema.Table`
+        for constructor arguments.
+
+        """
+        # __init__ is overridden to prevent __new__ from
+        # calling the superclass constructor.
+
+    def _init(self, name, metadata, *args, **kwargs):
+        super(Table, self).__init__(
+            quoted_name(name, kwargs.pop("quote", None))
+        )
+        self.metadata = metadata
+
+        self.schema = kwargs.pop("schema", None)
+        if self.schema is None:
+            self.schema = metadata.schema
+        elif self.schema is BLANK_SCHEMA:
+            self.schema = None
+        else:
+            quote_schema = kwargs.pop("quote_schema", None)
+            self.schema = quoted_name(self.schema, quote_schema)
+
+        self.indexes = set()
+        self.constraints = set()
+        PrimaryKeyConstraint(
+            _implicit_generated=True
+        )._set_parent_with_dispatch(self)
+        self.foreign_keys = set()
+        self._extra_dependencies = set()
+        if self.schema is not None:
+            self.fullname = "%s.%s" % (self.schema, self.name)
+        else:
+            self.fullname = self.name
+
+        autoload_with = kwargs.pop("autoload_with", None)
+        autoload = kwargs.pop("autoload", autoload_with is not None)
+        # this argument is only used with _init_existing()
+        kwargs.pop("autoload_replace", True)
+        keep_existing = kwargs.pop("keep_existing", False)
+        extend_existing = kwargs.pop("extend_existing", False)
+        _extend_on = kwargs.pop("_extend_on", None)
+
+        resolve_fks = kwargs.pop("resolve_fks", True)
+        include_columns = kwargs.pop("include_columns", None)
+
+        self.implicit_returning = kwargs.pop("implicit_returning", True)
+
+        self.comment = kwargs.pop("comment", None)
+
+        if "info" in kwargs:
+            self.info = kwargs.pop("info")
+        if "listeners" in kwargs:
+            listeners = kwargs.pop("listeners")
+            for evt, fn in listeners:
+                event.listen(self, evt, fn)
+
+        self._prefixes = kwargs.pop("prefixes", None) or []
+
+        self._extra_kwargs(**kwargs)
+
+        # load column definitions from the database if 'autoload' is defined
+        # we do it after the table is in the singleton dictionary to support
+        # circular foreign keys
+        if autoload:
+            self._autoload(
+                metadata,
+                autoload_with,
+                include_columns,
+                _extend_on=_extend_on,
+                resolve_fks=resolve_fks,
+            )
+
+        # initialize all the column, etc. objects.  done after reflection to
+        # allow user-overrides
+
+        self._init_items(
+            *args,
+            allow_replacements=extend_existing or keep_existing or autoload
+        )
+
+    def _autoload(
+        self,
+        metadata,
+        autoload_with,
+        include_columns,
+        exclude_columns=(),
+        resolve_fks=True,
+        _extend_on=None,
+    ):
+        if autoload_with is None:
+            autoload_with = _bind_or_error(
+                metadata,
+                msg="No engine is bound to this Table's MetaData. "
+                "Pass an engine to the Table via "
+                "autoload_with=<someengine_or_connection>",
+            )
+
+        insp = inspection.inspect(autoload_with)
+        with insp._inspection_context() as conn_insp:
+            conn_insp.reflect_table(
+                self,
+                include_columns,
+                exclude_columns,
+                resolve_fks,
+                _extend_on=_extend_on,
+            )
+
+    @property
+    def _sorted_constraints(self):
+        """Return the set of constraints as a list, sorted by creation
+        order.
+
+        """
+        return sorted(self.constraints, key=lambda c: c._creation_order)
+
+    @property
+    def foreign_key_constraints(self):
+        """:class:`_schema.ForeignKeyConstraint` objects referred to by this
+        :class:`_schema.Table`.
+
+        This list is produced from the collection of
+        :class:`_schema.ForeignKey`
+        objects currently associated.
+
+
+        .. seealso::
+
+            :attr:`_schema.Table.constraints`
+
+            :attr:`_schema.Table.foreign_keys`
+
+            :attr:`_schema.Table.indexes`
+
+        """
+        return set(fkc.constraint for fkc in self.foreign_keys)
+
+    def _init_existing(self, *args, **kwargs):
+        autoload_with = kwargs.pop("autoload_with", None)
+        autoload = kwargs.pop("autoload", autoload_with is not None)
+        autoload_replace = kwargs.pop("autoload_replace", True)
+        schema = kwargs.pop("schema", None)
+        _extend_on = kwargs.pop("_extend_on", None)
+        # these arguments are only used with _init()
+        kwargs.pop("extend_existing", False)
+        kwargs.pop("keep_existing", False)
+
+        if schema and schema != self.schema:
+            raise exc.ArgumentError(
+                "Can't change schema of existing table from '%s' to '%s'",
+                (self.schema, schema),
+            )
+
+        include_columns = kwargs.pop("include_columns", None)
+        if include_columns is not None:
+            for c in self.c:
+                if c.name not in include_columns:
+                    self._columns.remove(c)
+
+        resolve_fks = kwargs.pop("resolve_fks", True)
+
+        for key in ("quote", "quote_schema"):
+            if key in kwargs:
+                raise exc.ArgumentError(
+                    "Can't redefine 'quote' or 'quote_schema' arguments"
+                )
+
+        # update `self` with these kwargs, if provided
+        self.comment = kwargs.pop("comment", self.comment)
+        self.implicit_returning = kwargs.pop(
+            "implicit_returning", self.implicit_returning
+        )
+        self.info = kwargs.pop("info", self.info)
+
+        if autoload:
+            if not autoload_replace:
+                # don't replace columns already present.
+                # we'd like to do this for constraints also however we don't
+                # have simple de-duping for unnamed constraints.
+                exclude_columns = [c.name for c in self.c]
+            else:
+                exclude_columns = ()
+            self._autoload(
+                self.metadata,
+                autoload_with,
+                include_columns,
+                exclude_columns,
+                resolve_fks,
+                _extend_on=_extend_on,
+            )
+
+        self._extra_kwargs(**kwargs)
+        self._init_items(*args)
+
+    def _extra_kwargs(self, **kwargs):
+        self._validate_dialect_kwargs(kwargs)
+
+    def _init_collections(self):
+        pass
+
+    def _reset_exported(self):
+        pass
+
+    @property
+    def _autoincrement_column(self):
+        return self.primary_key._autoincrement_column
+
+    @property
+    def key(self):
+        """Return the 'key' for this :class:`_schema.Table`.
+
+        This value is used as the dictionary key within the
+        :attr:`_schema.MetaData.tables` collection.   It is typically the same
+        as that of :attr:`_schema.Table.name` for a table with no
+        :attr:`_schema.Table.schema`
+        set; otherwise it is typically of the form
+        ``schemaname.tablename``.
+
+        """
+        return _get_table_key(self.name, self.schema)
+
+    def __repr__(self):
+        return "Table(%s)" % ", ".join(
+            [repr(self.name)]
+            + [repr(self.metadata)]
+            + [repr(x) for x in self.columns]
+            + ["%s=%s" % (k, repr(getattr(self, k))) for k in ["schema"]]
+        )
+
+    def __str__(self):
+        return _get_table_key(self.description, self.schema)
+
+    @property
+    def bind(self):
+        """Return the connectable associated with this Table."""
+
+        return self.metadata and self.metadata.bind or None
+
+    def add_is_dependent_on(self, table):
+        """Add a 'dependency' for this Table.
+
+        This is another Table object which must be created
+        first before this one can, or dropped after this one.
+
+        Usually, dependencies between tables are determined via
+        ForeignKey objects.   However, for other situations that
+        create dependencies outside of foreign keys (rules, inheriting),
+        this method can manually establish such a link.
+
+        """
+        self._extra_dependencies.add(table)
+
+    def append_column(self, column, replace_existing=False):
+        """Append a :class:`_schema.Column` to this :class:`_schema.Table`.
+
+        The "key" of the newly added :class:`_schema.Column`, i.e. the
+        value of its ``.key`` attribute, will then be available
+        in the ``.c`` collection of this :class:`_schema.Table`, and the
+        column definition will be included in any CREATE TABLE, SELECT,
+        UPDATE, etc. statements generated from this :class:`_schema.Table`
+        construct.
+
+        Note that this does **not** change the definition of the table
+        as it exists within any underlying database, assuming that
+        table has already been created in the database.   Relational
+        databases support the addition of columns to existing tables
+        using the SQL ALTER command, which would need to be
+        emitted for an already-existing table that doesn't contain
+        the newly added column.
+
+        :param replace_existing: When ``True``, allows replacing existing
+            columns. When ``False``, the default, an warning will be raised
+            if a column with the same ``.key`` already exists. A future
+            version of sqlalchemy will instead rise a warning.
+
+            .. versionadded:: 1.4.0
+        """
+
+        column._set_parent_with_dispatch(
+            self, allow_replacements=replace_existing
+        )
+
+    def append_constraint(self, constraint):
+        """Append a :class:`_schema.Constraint` to this
+        :class:`_schema.Table`.
+
+        This has the effect of the constraint being included in any
+        future CREATE TABLE statement, assuming specific DDL creation
+        events have not been associated with the given
+        :class:`_schema.Constraint` object.
+
+        Note that this does **not** produce the constraint within the
+        relational database automatically, for a table that already exists
+        in the database.   To add a constraint to an
+        existing relational database table, the SQL ALTER command must
+        be used.  SQLAlchemy also provides the
+        :class:`.AddConstraint` construct which can produce this SQL when
+        invoked as an executable clause.
+
+        """
+
+        constraint._set_parent_with_dispatch(self)
+
+    def _set_parent(self, metadata, **kw):
+        metadata._add_table(self.name, self.schema, self)
+        self.metadata = metadata
+
+    @util.deprecated(
+        "1.4",
+        "The :meth:`_schema.Table.exists` method is deprecated and will be "
+        "removed in a future release.  Please refer to "
+        ":meth:`_reflection.Inspector.has_table`.",
+    )
+    def exists(self, bind=None):
+        """Return True if this table exists."""
+
+        if bind is None:
+            bind = _bind_or_error(self)
+
+        insp = inspection.inspect(bind)
+        return insp.has_table(self.name, schema=self.schema)
+
+    def create(self, bind=None, checkfirst=False):
+        """Issue a ``CREATE`` statement for this
+        :class:`_schema.Table`, using the given :class:`.Connectable`
+        for connectivity.
+
+        .. note:: the "bind" argument will be required in
+           SQLAlchemy 2.0.
+
+        .. seealso::
+
+            :meth:`_schema.MetaData.create_all`.
+
+        """
+
+        if bind is None:
+            bind = _bind_or_error(self)
+        bind._run_ddl_visitor(ddl.SchemaGenerator, self, checkfirst=checkfirst)
+
+    def drop(self, bind=None, checkfirst=False):
+        """Issue a ``DROP`` statement for this
+        :class:`_schema.Table`, using the given :class:`.Connectable`
+        for connectivity.
+
+        .. note:: the "bind" argument will be required in
+           SQLAlchemy 2.0.
+
+        .. seealso::
+
+            :meth:`_schema.MetaData.drop_all`.
+
+        """
+        if bind is None:
+            bind = _bind_or_error(self)
+        bind._run_ddl_visitor(ddl.SchemaDropper, self, checkfirst=checkfirst)
+
+    @util.deprecated(
+        "1.4",
+        ":meth:`_schema.Table.tometadata` is renamed to "
+        ":meth:`_schema.Table.to_metadata`",
+    )
+    def tometadata(
+        self,
+        metadata,
+        schema=RETAIN_SCHEMA,
+        referred_schema_fn=None,
+        name=None,
+    ):
+        """Return a copy of this :class:`_schema.Table`
+        associated with a different
+        :class:`_schema.MetaData`.
+
+        See :meth:`_schema.Table.to_metadata` for a full description.
+
+        """
+        return self.to_metadata(
+            metadata,
+            schema=schema,
+            referred_schema_fn=referred_schema_fn,
+            name=name,
+        )
+
+    def to_metadata(
+        self,
+        metadata,
+        schema=RETAIN_SCHEMA,
+        referred_schema_fn=None,
+        name=None,
+    ):
+        """Return a copy of this :class:`_schema.Table` associated with a
+        different :class:`_schema.MetaData`.
+
+        E.g.::
+
+            m1 = MetaData()
+
+            user = Table('user', m1, Column('id', Integer, primary_key=True))
+
+            m2 = MetaData()
+            user_copy = user.to_metadata(m2)
+
+        .. versionchanged:: 1.4  The :meth:`_schema.Table.to_metadata` function
+           was renamed from :meth:`_schema.Table.tometadata`.
+
+
+        :param metadata: Target :class:`_schema.MetaData` object,
+         into which the
+         new :class:`_schema.Table` object will be created.
+
+        :param schema: optional string name indicating the target schema.
+         Defaults to the special symbol :attr:`.RETAIN_SCHEMA` which indicates
+         that no change to the schema name should be made in the new
+         :class:`_schema.Table`.  If set to a string name, the new
+         :class:`_schema.Table`
+         will have this new name as the ``.schema``.  If set to ``None``, the
+         schema will be set to that of the schema set on the target
+         :class:`_schema.MetaData`, which is typically ``None`` as well,
+         unless
+         set explicitly::
+
+            m2 = MetaData(schema='newschema')
+
+            # user_copy_one will have "newschema" as the schema name
+            user_copy_one = user.to_metadata(m2, schema=None)
+
+            m3 = MetaData()  # schema defaults to None
+
+            # user_copy_two will have None as the schema name
+            user_copy_two = user.to_metadata(m3, schema=None)
+
+        :param referred_schema_fn: optional callable which can be supplied
+         in order to provide for the schema name that should be assigned
+         to the referenced table of a :class:`_schema.ForeignKeyConstraint`.
+         The callable accepts this parent :class:`_schema.Table`, the
+         target schema that we are changing to, the
+         :class:`_schema.ForeignKeyConstraint` object, and the existing
+         "target schema" of that constraint.  The function should return the
+         string schema name that should be applied.    To reset the schema
+         to "none", return the symbol :data:`.BLANK_SCHEMA`.  To effect no
+         change, return ``None`` or :data:`.RETAIN_SCHEMA`.
+
+         .. versionchanged:: 1.4.33  The ``referred_schema_fn`` function
+            may return the :data:`.BLANK_SCHEMA` or :data:`.RETAIN_SCHEMA`
+            symbols.
+
+         E.g.::
+
+                def referred_schema_fn(table, to_schema,
+                                                constraint, referred_schema):
+                    if referred_schema == 'base_tables':
+                        return referred_schema
+                    else:
+                        return to_schema
+
+                new_table = table.to_metadata(m2, schema="alt_schema",
+                                        referred_schema_fn=referred_schema_fn)
+
+         .. versionadded:: 0.9.2
+
+        :param name: optional string name indicating the target table name.
+         If not specified or None, the table name is retained.  This allows
+         a :class:`_schema.Table` to be copied to the same
+         :class:`_schema.MetaData` target
+         with a new name.
+
+         .. versionadded:: 1.0.0
+
+        """
+        if name is None:
+            name = self.name
+        if schema is RETAIN_SCHEMA:
+            schema = self.schema
+        elif schema is None:
+            schema = metadata.schema
+        key = _get_table_key(name, schema)
+        if key in metadata.tables:
+            util.warn(
+                "Table '%s' already exists within the given "
+                "MetaData - not copying." % self.description
+            )
+            return metadata.tables[key]
+
+        args = []
+        for c in self.columns:
+            args.append(c._copy(schema=schema))
+        table = Table(
+            name,
+            metadata,
+            schema=schema,
+            comment=self.comment,
+            *args,
+            **self.kwargs
+        )
+        for c in self.constraints:
+            if isinstance(c, ForeignKeyConstraint):
+                referred_schema = c._referred_schema
+                if referred_schema_fn:
+                    fk_constraint_schema = referred_schema_fn(
+                        self, schema, c, referred_schema
+                    )
+                else:
+                    fk_constraint_schema = (
+                        schema if referred_schema == self.schema else None
+                    )
+                table.append_constraint(
+                    c._copy(schema=fk_constraint_schema, target_table=table)
+                )
+            elif not c._type_bound:
+                # skip unique constraints that would be generated
+                # by the 'unique' flag on Column
+                if c._column_flag:
+                    continue
+
+                table.append_constraint(
+                    c._copy(schema=schema, target_table=table)
+                )
+        for index in self.indexes:
+            # skip indexes that would be generated
+            # by the 'index' flag on Column
+            if index._column_flag:
+                continue
+            Index(
+                index.name,
+                unique=index.unique,
+                *[
+                    _copy_expression(expr, self, table)
+                    for expr in index.expressions
+                ],
+                _table=table,
+                **index.kwargs
+            )
+        return self._schema_item_copy(table)
+
+
+class Column(DialectKWArgs, SchemaItem, ColumnClause):
+    """Represents a column in a database table."""
+
+    __visit_name__ = "column"
+
+    inherit_cache = True
+
+    def __init__(self, *args, **kwargs):
+        r"""
+        Construct a new ``Column`` object.
+
+        :param name: The name of this column as represented in the database.
+          This argument may be the first positional argument, or specified
+          via keyword.
+
+          Names which contain no upper case characters
+          will be treated as case insensitive names, and will not be quoted
+          unless they are a reserved word.  Names with any number of upper
+          case characters will be quoted and sent exactly.  Note that this
+          behavior applies even for databases which standardize upper
+          case names as case insensitive such as Oracle.
+
+          The name field may be omitted at construction time and applied
+          later, at any time before the Column is associated with a
+          :class:`_schema.Table`.  This is to support convenient
+          usage within the :mod:`~sqlalchemy.ext.declarative` extension.
+
+        :param type\_: The column's type, indicated using an instance which
+          subclasses :class:`~sqlalchemy.types.TypeEngine`.  If no arguments
+          are required for the type, the class of the type can be sent
+          as well, e.g.::
+
+            # use a type with arguments
+            Column('data', String(50))
+
+            # use no arguments
+            Column('level', Integer)
+
+          The ``type`` argument may be the second positional argument
+          or specified by keyword.
+
+          If the ``type`` is ``None`` or is omitted, it will first default to
+          the special type :class:`.NullType`.  If and when this
+          :class:`_schema.Column` is made to refer to another column using
+          :class:`_schema.ForeignKey` and/or
+          :class:`_schema.ForeignKeyConstraint`, the type
+          of the remote-referenced column will be copied to this column as
+          well, at the moment that the foreign key is resolved against that
+          remote :class:`_schema.Column` object.
+
+          .. versionchanged:: 0.9.0
+            Support for propagation of type to a :class:`_schema.Column`
+            from its
+            :class:`_schema.ForeignKey` object has been improved and should be
+            more reliable and timely.
+
+        :param \*args: Additional positional arguments include various
+          :class:`.SchemaItem` derived constructs which will be applied
+          as options to the column.  These include instances of
+          :class:`.Constraint`, :class:`_schema.ForeignKey`,
+          :class:`.ColumnDefault`, :class:`.Sequence`, :class:`.Computed`
+          :class:`.Identity`.  In some cases an
+          equivalent keyword argument is available such as ``server_default``,
+          ``default`` and ``unique``.
+
+        :param autoincrement: Set up "auto increment" semantics for an
+          **integer primary key column with no foreign key dependencies**
+          (see later in this docstring for a more specific definition).
+          This may influence the :term:`DDL` that will be emitted for
+          this column during a table create, as well as how the column
+          will be considered when INSERT statements are compiled and
+          executed.
+
+          The default value is the string ``"auto"``,
+          which indicates that a single-column (i.e. non-composite) primary key
+          that is of an INTEGER type with no other client-side or server-side
+          default constructs indicated should receive auto increment semantics
+          automatically. Other values include ``True`` (force this column to
+          have auto-increment semantics for a :term:`composite primary key` as
+          well), ``False`` (this column should never have auto-increment
+          semantics), and the string ``"ignore_fk"`` (special-case for foreign
+          key columns, see below).
+
+          The term "auto increment semantics" refers both to the kind of DDL
+          that will be emitted for the column within a CREATE TABLE statement,
+          when methods such as :meth:`.MetaData.create_all` and
+          :meth:`.Table.create` are invoked, as well as how the column will be
+          considered when an INSERT statement is compiled and emitted to the
+          database:
+
+          * **DDL rendering** (i.e. :meth:`.MetaData.create_all`,
+            :meth:`.Table.create`): When used on a :class:`.Column` that has
+            no other
+            default-generating construct associated with it (such as a
+            :class:`.Sequence` or :class:`.Identity` construct), the parameter
+            will imply that database-specific keywords such as PostgreSQL
+            ``SERIAL``, MySQL ``AUTO_INCREMENT``, or ``IDENTITY`` on SQL Server
+            should also be rendered.  Not every database backend has an
+            "implied" default generator available; for example the Oracle
+            backend always needs an explicit construct such as
+            :class:`.Identity` to be included with a :class:`.Column` in order
+            for the DDL rendered to include auto-generating constructs to also
+            be produced in the database.
+
+          * **INSERT semantics** (i.e. when a :func:`_sql.insert` construct is
+            compiled into a SQL string and is then executed on a database using
+            :meth:`_engine.Connection.execute` or equivalent): A single-row
+            INSERT statement will be known to produce a new integer primary key
+            value automatically for this column, which will be accessible
+            after the statement is invoked via the
+            :attr:`.CursorResult.inserted_primary_key` attribute upon the
+            :class:`_result.Result` object.   This also applies towards use of the
+            ORM when ORM-mapped objects are persisted to the database,
+            indicating that a new integer primary key will be available to
+            become part of the :term:`identity key` for that object.  This
+            behavior takes place regardless of what DDL constructs are
+            associated with the :class:`_schema.Column` and is independent
+            of the "DDL Rendering" behavior discussed in the previous note
+            above.
+
+          The parameter may be set to ``True`` to indicate that a column which
+          is part of a composite (i.e. multi-column) primary key should
+          have autoincrement semantics, though note that only one column
+          within a primary key may have this setting.    It can also
+          be set to ``True`` to indicate autoincrement semantics on a
+          column that has a client-side or server-side default configured,
+          however note that not all dialects can accommodate all styles
+          of default as an "autoincrement".  It can also be
+          set to ``False`` on a single-column primary key that has a
+          datatype of INTEGER in order to disable auto increment semantics
+          for that column.
+
+          .. versionchanged:: 1.1 The autoincrement flag now defaults to
+             ``"auto"`` which indicates autoincrement semantics by default
+             for single-column integer primary keys only; for composite
+             (multi-column) primary keys, autoincrement is never implicitly
+             enabled; as always, ``autoincrement=True`` will allow for
+             at most one of those columns to be an "autoincrement" column.
+             ``autoincrement=True`` may also be set on a
+             :class:`_schema.Column`
+             that has an explicit client-side or server-side default,
+             subject to limitations of the backend database and dialect.
+
+          The setting *only* has an effect for columns which are:
+
+          * Integer derived (i.e. INT, SMALLINT, BIGINT).
+
+          * Part of the primary key
+
+          * Not referring to another column via :class:`_schema.ForeignKey`,
+            unless
+            the value is specified as ``'ignore_fk'``::
+
+                # turn on autoincrement for this column despite
+                # the ForeignKey()
+                Column('id', ForeignKey('other.id'),
+                            primary_key=True, autoincrement='ignore_fk')
+
+          It is typically not desirable to have "autoincrement" enabled on a
+          column that refers to another via foreign key, as such a column is
+          required to refer to a value that originates from elsewhere.
+
+          The setting has these effects on columns that meet the
+          above criteria:
+
+          * DDL issued for the column, if the column does not already include
+            a default generating construct supported by the backend such as
+            :class:`.Identity`, will include database-specific
+            keywords intended to signify this column as an
+            "autoincrement" column for specific backends.   Behavior for
+            primary SQLAlchemy dialects includes:
+
+            * AUTO INCREMENT on MySQL and MariaDB
+            * SERIAL on PostgreSQL
+            * IDENTITY on MS-SQL - this occurs even without the
+              :class:`.Identity` construct as the
+              :paramref:`.Column.autoincrement` parameter pre-dates this
+              construct.
+            * SQLite - SQLite integer primary key columns are implicitly
+              "auto incrementing" and no additional keywords are rendered;
+              to render the special SQLite keyword ``AUTOINCREMENT``
+              is not included as this is unnecessary and not recommended
+              by the database vendor.  See the section
+              :ref:`sqlite_autoincrement` for more background.
+            * Oracle - The Oracle dialect has no default "autoincrement"
+              feature available at this time, instead the :class:`.Identity`
+              construct is recommended to achieve this (the :class:`.Sequence`
+              construct may also be used).
+            * Third-party dialects - consult those dialects' documentation
+              for details on their specific behaviors.
+
+          * When a single-row :func:`_sql.insert` construct is compiled and
+            executed, which does not set the :meth:`_sql.Insert.inline`
+            modifier, newly generated primary key values for this column
+            will be automatically retrieved upon statement execution
+            using a method specific to the database driver in use:
+
+            * MySQL, SQLite - calling upon ``cursor.lastrowid()``
+              (see
+              `https://www.python.org/dev/peps/pep-0249/#lastrowid
+              <https://www.python.org/dev/peps/pep-0249/#lastrowid>`_)
+            * PostgreSQL, SQL Server, Oracle - use RETURNING or an equivalent
+              construct when rendering an INSERT statement, and then retrieving
+              the newly generated primary key values after execution
+            * PostgreSQL, Oracle for :class:`_schema.Table` objects that
+              set :paramref:`_schema.Table.implicit_returning` to False -
+              for a :class:`.Sequence` only, the :class:`.Sequence` is invoked
+              explicitly before the INSERT statement takes place so that the
+              newly generated primary key value is available to the client
+            * SQL Server for :class:`_schema.Table` objects that
+              set :paramref:`_schema.Table.implicit_returning` to False -
+              the ``SELECT scope_identity()`` construct is used after the
+              INSERT statement is invoked to retrieve the newly generated
+              primary key value.
+            * Third-party dialects - consult those dialects' documentation
+              for details on their specific behaviors.
+
+          * For multiple-row :func:`_sql.insert` constructs invoked with
+            a list of parameters (i.e. "executemany" semantics), primary-key
+            retrieving behaviors are generally disabled, however there may
+            be special APIs that may be used to retrieve lists of new
+            primary key values for an "executemany", such as the psycopg2
+            "fast insertmany" feature.  Such features are very new and
+            may not yet be well covered in documentation.
+
+        :param default: A scalar, Python callable, or
+            :class:`_expression.ColumnElement` expression representing the
+            *default value* for this column, which will be invoked upon insert
+            if this column is otherwise not specified in the VALUES clause of
+            the insert. This is a shortcut to using :class:`.ColumnDefault` as
+            a positional argument; see that class for full detail on the
+            structure of the argument.
+
+            Contrast this argument to
+            :paramref:`_schema.Column.server_default`
+            which creates a default generator on the database side.
+
+            .. seealso::
+
+                :ref:`metadata_defaults_toplevel`
+
+        :param doc: optional String that can be used by the ORM or similar
+            to document attributes on the Python side.   This attribute does
+            **not** render SQL comments; use the
+            :paramref:`_schema.Column.comment`
+            parameter for this purpose.
+
+        :param key: An optional string identifier which will identify this
+            ``Column`` object on the :class:`_schema.Table`.
+            When a key is provided,
+            this is the only identifier referencing the ``Column`` within the
+            application, including ORM attribute mapping; the ``name`` field
+            is used only when rendering SQL.
+
+        :param index: When ``True``, indicates that a :class:`_schema.Index`
+            construct will be automatically generated for this
+            :class:`_schema.Column`, which will result in a "CREATE INDEX"
+            statement being emitted for the :class:`_schema.Table` when the DDL
+            create operation is invoked.
+
+            Using this flag is equivalent to making use of the
+            :class:`_schema.Index` construct explicitly at the level of the
+            :class:`_schema.Table` construct itself::
+
+                Table(
+                    "some_table",
+                    metadata,
+                    Column("x", Integer),
+                    Index("ix_some_table_x", "x")
+                )
+
+            To add the :paramref:`_schema.Index.unique` flag to the
+            :class:`_schema.Index`, set both the
+            :paramref:`_schema.Column.unique` and
+            :paramref:`_schema.Column.index` flags to True simultaneously,
+            which will have the effect of rendering the "CREATE UNIQUE INDEX"
+            DDL instruction instead of "CREATE INDEX".
+
+            The name of the index is generated using the
+            :ref:`default naming convention <constraint_default_naming_convention>`
+            which for the :class:`_schema.Index` construct is of the form
+            ``ix_<tablename>_<columnname>``.
+
+            As this flag is intended only as a convenience for the common case
+            of adding a single-column, default configured index to a table
+            definition, explicit use of the :class:`_schema.Index` construct
+            should be preferred for most use cases, including composite indexes
+            that encompass more than one column, indexes with SQL expressions
+            or ordering, backend-specific index configuration options, and
+            indexes that use a specific name.
+
+            .. note:: the :attr:`_schema.Column.index` attribute on
+               :class:`_schema.Column`
+               **does not indicate** if this column is indexed or not, only
+               if this flag was explicitly set here.  To view indexes on
+               a column, view the :attr:`_schema.Table.indexes` collection
+               or use :meth:`_reflection.Inspector.get_indexes`.
+
+            .. seealso::
+
+                :ref:`schema_indexes`
+
+                :ref:`constraint_naming_conventions`
+
+                :paramref:`_schema.Column.unique`
+
+        :param info: Optional data dictionary which will be populated into the
+            :attr:`.SchemaItem.info` attribute of this object.
+
+        :param nullable: When set to ``False``, will cause the "NOT NULL"
+            phrase to be added when generating DDL for the column.   When
+            ``True``, will normally generate nothing (in SQL this defaults to
+            "NULL"), except in some very specific backend-specific edge cases
+            where "NULL" may render explicitly.
+            Defaults to ``True`` unless :paramref:`_schema.Column.primary_key`
+            is also ``True`` or the column specifies a :class:`_sql.Identity`,
+            in which case it defaults to ``False``.
+            This parameter is only used when issuing CREATE TABLE statements.
+
+            .. note::
+
+                When the column specifies a :class:`_sql.Identity` this
+                parameter is in general ignored by the DDL compiler. The
+                PostgreSQL database allows nullable identity column by
+                setting this parameter to ``True`` explicitly.
+
+        :param onupdate: A scalar, Python callable, or
+            :class:`~sqlalchemy.sql.expression.ClauseElement` representing a
+            default value to be applied to the column within UPDATE
+            statements, which will be invoked upon update if this column is not
+            present in the SET clause of the update. This is a shortcut to
+            using :class:`.ColumnDefault` as a positional argument with
+            ``for_update=True``.
+
+            .. seealso::
+
+                :ref:`metadata_defaults` - complete discussion of onupdate
+
+        :param primary_key: If ``True``, marks this column as a primary key
+            column. Multiple columns can have this flag set to specify
+            composite primary keys. As an alternative, the primary key of a
+            :class:`_schema.Table` can be specified via an explicit
+            :class:`.PrimaryKeyConstraint` object.
+
+        :param server_default: A :class:`.FetchedValue` instance, str, Unicode
+            or :func:`~sqlalchemy.sql.expression.text` construct representing
+            the DDL DEFAULT value for the column.
+
+            String types will be emitted as-is, surrounded by single quotes::
+
+                Column('x', Text, server_default="val")
+
+                x TEXT DEFAULT 'val'
+
+            A :func:`~sqlalchemy.sql.expression.text` expression will be
+            rendered as-is, without quotes::
+
+                Column('y', DateTime, server_default=text('NOW()'))
+
+                y DATETIME DEFAULT NOW()
+
+            Strings and text() will be converted into a
+            :class:`.DefaultClause` object upon initialization.
+
+            This parameter can also accept complex combinations of contextually
+            valid SQLAlchemy expressions or constructs::
+
+                from sqlalchemy import create_engine
+                from sqlalchemy import Table, Column, MetaData, ARRAY, Text
+                from sqlalchemy.dialects.postgresql import array
+
+                engine = create_engine(
+                    'postgresql://scott:tiger@localhost/mydatabase'
+                )
+                metadata_obj = MetaData()
+                tbl = Table(
+                        "foo",
+                        metadata_obj,
+                        Column("bar",
+                               ARRAY(Text),
+                               server_default=array(["biz", "bang", "bash"])
+                               )
+                )
+                metadata_obj.create_all(engine)
+
+            The above results in a table created with the following SQL::
+
+                CREATE TABLE foo (
+                    bar TEXT[] DEFAULT ARRAY['biz', 'bang', 'bash']
+                )
+
+            Use :class:`.FetchedValue` to indicate that an already-existing
+            column will generate a default value on the database side which
+            will be available to SQLAlchemy for post-fetch after inserts. This
+            construct does not specify any DDL and the implementation is left
+            to the database, such as via a trigger.
+
+            .. seealso::
+
+                :ref:`server_defaults` - complete discussion of server side
+                defaults
+
+        :param server_onupdate: A :class:`.FetchedValue` instance
+            representing a database-side default generation function,
+            such as a trigger. This
+            indicates to SQLAlchemy that a newly generated value will be
+            available after updates. This construct does not actually
+            implement any kind of generation function within the database,
+            which instead must be specified separately.
+
+
+            .. warning:: This directive **does not** currently produce MySQL's
+               "ON UPDATE CURRENT_TIMESTAMP()" clause.  See
+               :ref:`mysql_timestamp_onupdate` for background on how to
+               produce this clause.
+
+            .. seealso::
+
+                :ref:`triggered_columns`
+
+        :param quote: Force quoting of this column's name on or off,
+             corresponding to ``True`` or ``False``. When left at its default
+             of ``None``, the column identifier will be quoted according to
+             whether the name is case sensitive (identifiers with at least one
+             upper case character are treated as case sensitive), or if it's a
+             reserved word. This flag is only needed to force quoting of a
+             reserved word which is not known by the SQLAlchemy dialect.
+
+        :param unique: When ``True``, and the :paramref:`_schema.Column.index`
+            parameter is left at its default value of ``False``,
+            indicates that a :class:`_schema.UniqueConstraint`
+            construct will be automatically generated for this
+            :class:`_schema.Column`,
+            which will result in a "UNIQUE CONSTRAINT" clause referring
+            to this column being included
+            in the ``CREATE TABLE`` statement emitted, when the DDL create
+            operation for the :class:`_schema.Table` object is invoked.
+
+            When this flag is ``True`` while the
+            :paramref:`_schema.Column.index` parameter is simultaneously
+            set to ``True``, the effect instead is that a
+            :class:`_schema.Index` construct which includes the
+            :paramref:`_schema.Index.unique` parameter set to ``True``
+            is generated.  See the documentation for
+            :paramref:`_schema.Column.index` for additional detail.
+
+            Using this flag is equivalent to making use of the
+            :class:`_schema.UniqueConstraint` construct explicitly at the
+            level of the :class:`_schema.Table` construct itself::
+
+                Table(
+                    "some_table",
+                    metadata,
+                    Column("x", Integer),
+                    UniqueConstraint("x")
+                )
+
+            The :paramref:`_schema.UniqueConstraint.name` parameter
+            of the unique constraint object is left at its default value
+            of ``None``; in the absence of a :ref:`naming convention <constraint_naming_conventions>`
+            for the enclosing :class:`_schema.MetaData`, the UNIQUE CONSTRAINT
+            construct will be emitted as unnamed, which typically invokes
+            a database-specific naming convention to take place.
+
+            As this flag is intended only as a convenience for the common case
+            of adding a single-column, default configured unique constraint to a table
+            definition, explicit use of the :class:`_schema.UniqueConstraint` construct
+            should be preferred for most use cases, including composite constraints
+            that encompass more than one column, backend-specific index configuration options, and
+            constraints that use a specific name.
+
+            .. note:: the :attr:`_schema.Column.unique` attribute on
+                :class:`_schema.Column`
+                **does not indicate** if this column has a unique constraint or
+                not, only if this flag was explicitly set here.  To view
+                indexes and unique constraints that may involve this column,
+                view the
+                :attr:`_schema.Table.indexes` and/or
+                :attr:`_schema.Table.constraints` collections or use
+                :meth:`_reflection.Inspector.get_indexes` and/or
+                :meth:`_reflection.Inspector.get_unique_constraints`
+
+            .. seealso::
+
+                :ref:`schema_unique_constraint`
+
+                :ref:`constraint_naming_conventions`
+
+                :paramref:`_schema.Column.index`
+
+        :param system: When ``True``, indicates this is a "system" column,
+             that is a column which is automatically made available by the
+             database, and should not be included in the columns list for a
+             ``CREATE TABLE`` statement.
+
+             For more elaborate scenarios where columns should be
+             conditionally rendered differently on different backends,
+             consider custom compilation rules for :class:`.CreateColumn`.
+
+        :param comment: Optional string that will render an SQL comment on
+             table creation.
+
+             .. versionadded:: 1.2 Added the
+                :paramref:`_schema.Column.comment`
+                parameter to :class:`_schema.Column`.
+
+
+        """  # noqa: E501, RST201, RST202
+
+        name = kwargs.pop("name", None)
+        type_ = kwargs.pop("type_", None)
+        args = list(args)
+        if args:
+            if isinstance(args[0], util.string_types):
+                if name is not None:
+                    raise exc.ArgumentError(
+                        "May not pass name positionally and as a keyword."
+                    )
+                name = args.pop(0)
+        if args:
+            coltype = args[0]
+
+            if hasattr(coltype, "_sqla_type"):
+                if type_ is not None:
+                    raise exc.ArgumentError(
+                        "May not pass type_ positionally and as a keyword."
+                    )
+                type_ = args.pop(0)
+
+        if name is not None:
+            name = quoted_name(name, kwargs.pop("quote", None))
+        elif "quote" in kwargs:
+            raise exc.ArgumentError(
+                "Explicit 'name' is required when " "sending 'quote' argument"
+            )
+
+        super(Column, self).__init__(name, type_)
+        self.key = kwargs.pop("key", name)
+        self.primary_key = primary_key = kwargs.pop("primary_key", False)
+
+        self._user_defined_nullable = udn = kwargs.pop(
+            "nullable", NULL_UNSPECIFIED
+        )
+
+        if udn is not NULL_UNSPECIFIED:
+            self.nullable = udn
+        else:
+            self.nullable = not primary_key
+
+        self.default = kwargs.pop("default", None)
+        self.server_default = kwargs.pop("server_default", None)
+        self.server_onupdate = kwargs.pop("server_onupdate", None)
+
+        # these default to None because .index and .unique is *not*
+        # an informational flag about Column - there can still be an
+        # Index or UniqueConstraint referring to this Column.
+        self.index = kwargs.pop("index", None)
+        self.unique = kwargs.pop("unique", None)
+
+        self.system = kwargs.pop("system", False)
+        self.doc = kwargs.pop("doc", None)
+        self.onupdate = kwargs.pop("onupdate", None)
+        self.autoincrement = kwargs.pop("autoincrement", "auto")
+        self.constraints = set()
+        self.foreign_keys = set()
+        self.comment = kwargs.pop("comment", None)
+        self.computed = None
+        self.identity = None
+
+        # check if this Column is proxying another column
+        if "_proxies" in kwargs:
+            self._proxies = kwargs.pop("_proxies")
+        # otherwise, add DDL-related events
+        elif isinstance(self.type, SchemaEventTarget):
+            self.type._set_parent_with_dispatch(self)
+
+        if self.default is not None:
+            if isinstance(self.default, (ColumnDefault, Sequence)):
+                args.append(self.default)
+            else:
+                if getattr(self.type, "_warn_on_bytestring", False):
+                    if isinstance(self.default, util.binary_type):
+                        util.warn(
+                            "Unicode column '%s' has non-unicode "
+                            "default value %r specified."
+                            % (self.key, self.default)
+                        )
+                args.append(ColumnDefault(self.default))
+
+        if self.server_default is not None:
+            if isinstance(self.server_default, FetchedValue):
+                args.append(self.server_default._as_for_update(False))
+            else:
+                args.append(DefaultClause(self.server_default))
+
+        if self.onupdate is not None:
+            if isinstance(self.onupdate, (ColumnDefault, Sequence)):
+                args.append(self.onupdate)
+            else:
+                args.append(ColumnDefault(self.onupdate, for_update=True))
+
+        if self.server_onupdate is not None:
+            if isinstance(self.server_onupdate, FetchedValue):
+                args.append(self.server_onupdate._as_for_update(True))
+            else:
+                args.append(
+                    DefaultClause(self.server_onupdate, for_update=True)
+                )
+        self._init_items(*args)
+
+        util.set_creation_order(self)
+
+        if "info" in kwargs:
+            self.info = kwargs.pop("info")
+
+        self._extra_kwargs(**kwargs)
+
+    foreign_keys = None
+    """A collection of all :class:`_schema.ForeignKey` marker objects
+       associated with this :class:`_schema.Column`.
+
+       Each object is a member of a :class:`_schema.Table`-wide
+       :class:`_schema.ForeignKeyConstraint`.
+
+       .. seealso::
+
+           :attr:`_schema.Table.foreign_keys`
+
+    """
+
+    index = None
+    """The value of the :paramref:`_schema.Column.index` parameter.
+
+       Does not indicate if this :class:`_schema.Column` is actually indexed
+       or not; use :attr:`_schema.Table.indexes`.
+
+       .. seealso::
+
+           :attr:`_schema.Table.indexes`
+    """
+
+    unique = None
+    """The value of the :paramref:`_schema.Column.unique` parameter.
+
+       Does not indicate if this :class:`_schema.Column` is actually subject to
+       a unique constraint or not; use :attr:`_schema.Table.indexes` and
+       :attr:`_schema.Table.constraints`.
+
+       .. seealso::
+
+           :attr:`_schema.Table.indexes`
+
+           :attr:`_schema.Table.constraints`.
+
+    """
+
+    def _extra_kwargs(self, **kwargs):
+        self._validate_dialect_kwargs(kwargs)
+
+    def __str__(self):
+        if self.name is None:
+            return "(no name)"
+        elif self.table is not None:
+            if self.table.named_with_column:
+                return self.table.description + "." + self.description
+            else:
+                return self.description
+        else:
+            return self.description
+
+    def references(self, column):
+        """Return True if this Column references the given column via foreign
+        key."""
+
+        for fk in self.foreign_keys:
+            if fk.column.proxy_set.intersection(column.proxy_set):
+                return True
+        else:
+            return False
+
+    def append_foreign_key(self, fk):
+        fk._set_parent_with_dispatch(self)
+
+    def __repr__(self):
+        kwarg = []
+        if self.key != self.name:
+            kwarg.append("key")
+        if self.primary_key:
+            kwarg.append("primary_key")
+        if not self.nullable:
+            kwarg.append("nullable")
+        if self.onupdate:
+            kwarg.append("onupdate")
+        if self.default:
+            kwarg.append("default")
+        if self.server_default:
+            kwarg.append("server_default")
+        if self.comment:
+            kwarg.append("comment")
+        return "Column(%s)" % ", ".join(
+            [repr(self.name)]
+            + [repr(self.type)]
+            + [repr(x) for x in self.foreign_keys if x is not None]
+            + [repr(x) for x in self.constraints]
+            + [
+                (
+                    self.table is not None
+                    and "table=<%s>" % self.table.description
+                    or "table=None"
+                )
+            ]
+            + ["%s=%s" % (k, repr(getattr(self, k))) for k in kwarg]
+        )
+
+    def _set_parent(self, table, allow_replacements=True):
+        if not self.name:
+            raise exc.ArgumentError(
+                "Column must be constructed with a non-blank name or "
+                "assign a non-blank .name before adding to a Table."
+            )
+
+        self._reset_memoizations()
+
+        if self.key is None:
+            self.key = self.name
+
+        existing = getattr(self, "table", None)
+        if existing is not None and existing is not table:
+            raise exc.ArgumentError(
+                "Column object '%s' already assigned to Table '%s'"
+                % (self.key, existing.description)
+            )
+
+        if self.key in table._columns:
+            col = table._columns.get(self.key)
+            if col is not self:
+                if not allow_replacements:
+                    util.warn_deprecated(
+                        "A column with name '%s' is already present "
+                        "in table '%s'. Please use method "
+                        ":meth:`_schema.Table.append_column` with the "
+                        "parameter ``replace_existing=True`` to replace an "
+                        "existing column." % (self.key, table.name),
+                        "1.4",
+                    )
+                for fk in col.foreign_keys:
+                    table.foreign_keys.remove(fk)
+                    if fk.constraint in table.constraints:
+                        # this might have been removed
+                        # already, if it's a composite constraint
+                        # and more than one col being replaced
+                        table.constraints.remove(fk.constraint)
+
+        table._columns.replace(self)
+
+        self.table = table
+
+        if self.primary_key:
+            table.primary_key._replace(self)
+        elif self.key in table.primary_key:
+            raise exc.ArgumentError(
+                "Trying to redefine primary-key column '%s' as a "
+                "non-primary-key column on table '%s'"
+                % (self.key, table.fullname)
+            )
+
+        if self.index:
+            if isinstance(self.index, util.string_types):
+                raise exc.ArgumentError(
+                    "The 'index' keyword argument on Column is boolean only. "
+                    "To create indexes with a specific name, create an "
+                    "explicit Index object external to the Table."
+                )
+            table.append_constraint(
+                Index(
+                    None, self.key, unique=bool(self.unique), _column_flag=True
+                )
+            )
+
+        elif self.unique:
+            if isinstance(self.unique, util.string_types):
+                raise exc.ArgumentError(
+                    "The 'unique' keyword argument on Column is boolean "
+                    "only. To create unique constraints or indexes with a "
+                    "specific name, append an explicit UniqueConstraint to "
+                    "the Table's list of elements, or create an explicit "
+                    "Index object external to the Table."
+                )
+            table.append_constraint(
+                UniqueConstraint(self.key, _column_flag=True)
+            )
+
+        self._setup_on_memoized_fks(lambda fk: fk._set_remote_table(table))
+
+        if self.identity and (
+            isinstance(self.default, Sequence)
+            or isinstance(self.onupdate, Sequence)
+        ):
+            raise exc.ArgumentError(
+                "An column cannot specify both Identity and Sequence."
+            )
+
+    def _setup_on_memoized_fks(self, fn):
+        fk_keys = [
+            ((self.table.key, self.key), False),
+            ((self.table.key, self.name), True),
+        ]
+        for fk_key, link_to_name in fk_keys:
+            if fk_key in self.table.metadata._fk_memos:
+                for fk in self.table.metadata._fk_memos[fk_key]:
+                    if fk.link_to_name is link_to_name:
+                        fn(fk)
+
+    def _on_table_attach(self, fn):
+        if self.table is not None:
+            fn(self, self.table)
+        else:
+            event.listen(self, "after_parent_attach", fn)
+
+    @util.deprecated(
+        "1.4",
+        "The :meth:`_schema.Column.copy` method is deprecated "
+        "and will be removed in a future release.",
+    )
+    def copy(self, **kw):
+        return self._copy(**kw)
+
+    def _copy(self, **kw):
+        """Create a copy of this ``Column``, uninitialized.
+
+        This is used in :meth:`_schema.Table.to_metadata`.
+
+        """
+
+        # Constraint objects plus non-constraint-bound ForeignKey objects
+        args = [
+            c._copy(**kw) for c in self.constraints if not c._type_bound
+        ] + [c._copy(**kw) for c in self.foreign_keys if not c.constraint]
+
+        # ticket #5276
+        column_kwargs = {}
+        for dialect_name in self.dialect_options:
+            dialect_options = self.dialect_options[dialect_name]._non_defaults
+            for (
+                dialect_option_key,
+                dialect_option_value,
+            ) in dialect_options.items():
+                column_kwargs[
+                    dialect_name + "_" + dialect_option_key
+                ] = dialect_option_value
+
+        server_default = self.server_default
+        server_onupdate = self.server_onupdate
+        if isinstance(server_default, (Computed, Identity)):
+            server_default = server_onupdate = None
+            args.append(self.server_default._copy(**kw))
+
+        type_ = self.type
+        if isinstance(type_, SchemaEventTarget):
+            type_ = type_.copy(**kw)
+
+        if self._user_defined_nullable is not NULL_UNSPECIFIED:
+            column_kwargs["nullable"] = self._user_defined_nullable
+
+        c = self._constructor(
+            name=self.name,
+            type_=type_,
+            key=self.key,
+            primary_key=self.primary_key,
+            unique=self.unique,
+            system=self.system,
+            # quote=self.quote,  # disabled 2013-08-27 (commit 031ef080)
+            index=self.index,
+            autoincrement=self.autoincrement,
+            default=self.default,
+            server_default=server_default,
+            onupdate=self.onupdate,
+            server_onupdate=server_onupdate,
+            doc=self.doc,
+            comment=self.comment,
+            *args,
+            **column_kwargs
+        )
+        return self._schema_item_copy(c)
+
+    def _make_proxy(
+        self, selectable, name=None, key=None, name_is_truncatable=False, **kw
+    ):
+        """Create a *proxy* for this column.
+
+        This is a copy of this ``Column`` referenced by a different parent
+        (such as an alias or select statement).  The column should
+        be used only in select scenarios, as its full DDL/default
+        information is not transferred.
+
+        """
+
+        fk = [
+            ForeignKey(
+                col if col is not None else f._colspec,
+                _unresolvable=col is None,
+                _constraint=f.constraint,
+            )
+            for f, col in [
+                (fk, fk._resolve_column(raiseerr=False))
+                for fk in self.foreign_keys
+            ]
+        ]
+
+        if name is None and self.name is None:
+            raise exc.InvalidRequestError(
+                "Cannot initialize a sub-selectable"
+                " with this Column object until its 'name' has "
+                "been assigned."
+            )
+        try:
+            c = self._constructor(
+                coercions.expect(
+                    roles.TruncatedLabelRole, name if name else self.name
+                )
+                if name_is_truncatable
+                else (name or self.name),
+                self.type,
+                # this may actually be ._proxy_key when the key is incoming
+                key=key if key else name if name else self.key,
+                primary_key=self.primary_key,
+                nullable=self.nullable,
+                _proxies=[self],
+                *fk
+            )
+        except TypeError as err:
+            util.raise_(
+                TypeError(
+                    "Could not create a copy of this %r object.  "
+                    "Ensure the class includes a _constructor() "
+                    "attribute or method which accepts the "
+                    "standard Column constructor arguments, or "
+                    "references the Column class itself." % self.__class__
+                ),
+                from_=err,
+            )
+
+        c.table = selectable
+        c._propagate_attrs = selectable._propagate_attrs
+        if selectable._is_clone_of is not None:
+            c._is_clone_of = selectable._is_clone_of.columns.get(c.key)
+        if self.primary_key:
+            selectable.primary_key.add(c)
+        if fk:
+            selectable.foreign_keys.update(fk)
+        return c.key, c
+
+
+class ForeignKey(DialectKWArgs, SchemaItem):
+    """Defines a dependency between two columns.
+
+    ``ForeignKey`` is specified as an argument to a :class:`_schema.Column`
+    object,
+    e.g.::
+
+        t = Table("remote_table", metadata,
+            Column("remote_id", ForeignKey("main_table.id"))
+        )
+
+    Note that ``ForeignKey`` is only a marker object that defines
+    a dependency between two columns.   The actual constraint
+    is in all cases represented by the :class:`_schema.ForeignKeyConstraint`
+    object.   This object will be generated automatically when
+    a ``ForeignKey`` is associated with a :class:`_schema.Column` which
+    in turn is associated with a :class:`_schema.Table`.   Conversely,
+    when :class:`_schema.ForeignKeyConstraint` is applied to a
+    :class:`_schema.Table`,
+    ``ForeignKey`` markers are automatically generated to be
+    present on each associated :class:`_schema.Column`, which are also
+    associated with the constraint object.
+
+    Note that you cannot define a "composite" foreign key constraint,
+    that is a constraint between a grouping of multiple parent/child
+    columns, using ``ForeignKey`` objects.   To define this grouping,
+    the :class:`_schema.ForeignKeyConstraint` object must be used, and applied
+    to the :class:`_schema.Table`.   The associated ``ForeignKey`` objects
+    are created automatically.
+
+    The ``ForeignKey`` objects associated with an individual
+    :class:`_schema.Column`
+    object are available in the `foreign_keys` collection
+    of that column.
+
+    Further examples of foreign key configuration are in
+    :ref:`metadata_foreignkeys`.
+
+    """
+
+    __visit_name__ = "foreign_key"
+
+    def __init__(
+        self,
+        column,
+        _constraint=None,
+        use_alter=False,
+        name=None,
+        onupdate=None,
+        ondelete=None,
+        deferrable=None,
+        initially=None,
+        link_to_name=False,
+        match=None,
+        info=None,
+        _unresolvable=False,
+        **dialect_kw
+    ):
+        r"""
+        Construct a column-level FOREIGN KEY.
+
+        The :class:`_schema.ForeignKey` object when constructed generates a
+        :class:`_schema.ForeignKeyConstraint`
+        which is associated with the parent
+        :class:`_schema.Table` object's collection of constraints.
+
+        :param column: A single target column for the key relationship. A
+            :class:`_schema.Column` object or a column name as a string:
+            ``tablename.columnkey`` or ``schema.tablename.columnkey``.
+            ``columnkey`` is the ``key`` which has been assigned to the column
+            (defaults to the column name itself), unless ``link_to_name`` is
+            ``True`` in which case the rendered name of the column is used.
+
+        :param name: Optional string. An in-database name for the key if
+            `constraint` is not provided.
+
+        :param onupdate: Optional string. If set, emit ON UPDATE <value> when
+            issuing DDL for this constraint. Typical values include CASCADE,
+            DELETE and RESTRICT.
+
+        :param ondelete: Optional string. If set, emit ON DELETE <value> when
+            issuing DDL for this constraint. Typical values include CASCADE,
+            DELETE and RESTRICT.
+
+        :param deferrable: Optional bool. If set, emit DEFERRABLE or NOT
+            DEFERRABLE when issuing DDL for this constraint.
+
+        :param initially: Optional string. If set, emit INITIALLY <value> when
+            issuing DDL for this constraint.
+
+        :param link_to_name: if True, the string name given in ``column`` is
+            the rendered name of the referenced column, not its locally
+            assigned ``key``.
+
+        :param use_alter: passed to the underlying
+            :class:`_schema.ForeignKeyConstraint`
+            to indicate the constraint should
+            be generated/dropped externally from the CREATE TABLE/ DROP TABLE
+            statement.  See :paramref:`_schema.ForeignKeyConstraint.use_alter`
+            for further description.
+
+            .. seealso::
+
+                :paramref:`_schema.ForeignKeyConstraint.use_alter`
+
+                :ref:`use_alter`
+
+        :param match: Optional string. If set, emit MATCH <value> when issuing
+            DDL for this constraint. Typical values include SIMPLE, PARTIAL
+            and FULL.
+
+        :param info: Optional data dictionary which will be populated into the
+            :attr:`.SchemaItem.info` attribute of this object.
+
+            .. versionadded:: 1.0.0
+
+        :param \**dialect_kw:  Additional keyword arguments are dialect
+            specific, and passed in the form ``<dialectname>_<argname>``.  The
+            arguments are ultimately handled by a corresponding
+            :class:`_schema.ForeignKeyConstraint`.
+            See the documentation regarding
+            an individual dialect at :ref:`dialect_toplevel` for detail on
+            documented arguments.
+
+            .. versionadded:: 0.9.2
+
+        """
+
+        self._colspec = coercions.expect(roles.DDLReferredColumnRole, column)
+        self._unresolvable = _unresolvable
+
+        if isinstance(self._colspec, util.string_types):
+            self._table_column = None
+        else:
+            self._table_column = self._colspec
+
+            if not isinstance(
+                self._table_column.table, (util.NoneType, TableClause)
+            ):
+                raise exc.ArgumentError(
+                    "ForeignKey received Column not bound "
+                    "to a Table, got: %r" % self._table_column.table
+                )
+
+        # the linked ForeignKeyConstraint.
+        # ForeignKey will create this when parent Column
+        # is attached to a Table, *or* ForeignKeyConstraint
+        # object passes itself in when creating ForeignKey
+        # markers.
+        self.constraint = _constraint
+        self.parent = None
+        self.use_alter = use_alter
+        self.name = name
+        self.onupdate = onupdate
+        self.ondelete = ondelete
+        self.deferrable = deferrable
+        self.initially = initially
+        self.link_to_name = link_to_name
+        self.match = match
+        if info:
+            self.info = info
+        self._unvalidated_dialect_kw = dialect_kw
+
+    def __repr__(self):
+        return "ForeignKey(%r)" % self._get_colspec()
+
+    @util.deprecated(
+        "1.4",
+        "The :meth:`_schema.ForeignKey.copy` method is deprecated "
+        "and will be removed in a future release.",
+    )
+    def copy(self, schema=None, **kw):
+        return self._copy(schema=schema, **kw)
+
+    def _copy(self, schema=None, **kw):
+        """Produce a copy of this :class:`_schema.ForeignKey` object.
+
+        The new :class:`_schema.ForeignKey` will not be bound
+        to any :class:`_schema.Column`.
+
+        This method is usually used by the internal
+        copy procedures of :class:`_schema.Column`, :class:`_schema.Table`,
+        and :class:`_schema.MetaData`.
+
+        :param schema: The returned :class:`_schema.ForeignKey` will
+          reference the original table and column name, qualified
+          by the given string schema name.
+
+        """
+
+        fk = ForeignKey(
+            self._get_colspec(schema=schema),
+            use_alter=self.use_alter,
+            name=self.name,
+            onupdate=self.onupdate,
+            ondelete=self.ondelete,
+            deferrable=self.deferrable,
+            initially=self.initially,
+            link_to_name=self.link_to_name,
+            match=self.match,
+            **self._unvalidated_dialect_kw
+        )
+        return self._schema_item_copy(fk)
+
+    def _get_colspec(self, schema=None, table_name=None):
+        """Return a string based 'column specification' for this
+        :class:`_schema.ForeignKey`.
+
+        This is usually the equivalent of the string-based "tablename.colname"
+        argument first passed to the object's constructor.
+
+        """
+        if schema not in (None, RETAIN_SCHEMA):
+            _schema, tname, colname = self._column_tokens
+            if table_name is not None:
+                tname = table_name
+            if schema is BLANK_SCHEMA:
+                return "%s.%s" % (tname, colname)
+            else:
+                return "%s.%s.%s" % (schema, tname, colname)
+        elif table_name:
+            schema, tname, colname = self._column_tokens
+            if schema:
+                return "%s.%s.%s" % (schema, table_name, colname)
+            else:
+                return "%s.%s" % (table_name, colname)
+        elif self._table_column is not None:
+            return "%s.%s" % (
+                self._table_column.table.fullname,
+                self._table_column.key,
+            )
+        else:
+            return self._colspec
+
+    @property
+    def _referred_schema(self):
+        return self._column_tokens[0]
+
+    def _table_key(self):
+        if self._table_column is not None:
+            if self._table_column.table is None:
+                return None
+            else:
+                return self._table_column.table.key
+        else:
+            schema, tname, colname = self._column_tokens
+            return _get_table_key(tname, schema)
+
+    target_fullname = property(_get_colspec)
+
+    def references(self, table):
+        """Return True if the given :class:`_schema.Table`
+        is referenced by this
+        :class:`_schema.ForeignKey`."""
+
+        return table.corresponding_column(self.column) is not None
+
+    def get_referent(self, table):
+        """Return the :class:`_schema.Column` in the given
+        :class:`_schema.Table`
+        referenced by this :class:`_schema.ForeignKey`.
+
+        Returns None if this :class:`_schema.ForeignKey`
+        does not reference the given
+        :class:`_schema.Table`.
+
+        """
+
+        return table.corresponding_column(self.column)
+
+    @util.memoized_property
+    def _column_tokens(self):
+        """parse a string-based _colspec into its component parts."""
+
+        m = self._get_colspec().split(".")
+        if m is None:
+            raise exc.ArgumentError(
+                "Invalid foreign key column specification: %s" % self._colspec
+            )
+        if len(m) == 1:
+            tname = m.pop()
+            colname = None
+        else:
+            colname = m.pop()
+            tname = m.pop()
+
+        # A FK between column 'bar' and table 'foo' can be
+        # specified as 'foo', 'foo.bar', 'dbo.foo.bar',
+        # 'otherdb.dbo.foo.bar'. Once we have the column name and
+        # the table name, treat everything else as the schema
+        # name. Some databases (e.g. Sybase) support
+        # inter-database foreign keys. See tickets#1341 and --
+        # indirectly related -- Ticket #594. This assumes that '.'
+        # will never appear *within* any component of the FK.
+
+        if len(m) > 0:
+            schema = ".".join(m)
+        else:
+            schema = None
+        return schema, tname, colname
+
+    def _resolve_col_tokens(self):
+        if self.parent is None:
+            raise exc.InvalidRequestError(
+                "this ForeignKey object does not yet have a "
+                "parent Column associated with it."
+            )
+
+        elif self.parent.table is None:
+            raise exc.InvalidRequestError(
+                "this ForeignKey's parent column is not yet associated "
+                "with a Table."
+            )
+
+        parenttable = self.parent.table
+
+        if self._unresolvable:
+            schema, tname, colname = self._column_tokens
+            tablekey = _get_table_key(tname, schema)
+            return parenttable, tablekey, colname
+
+        # assertion
+        # basically Column._make_proxy() sends the actual
+        # target Column to the ForeignKey object, so the
+        # string resolution here is never called.
+        for c in self.parent.base_columns:
+            if isinstance(c, Column):
+                assert c.table is parenttable
+                break
+        else:
+            assert False
+        ######################
+
+        schema, tname, colname = self._column_tokens
+
+        if schema is None and parenttable.metadata.schema is not None:
+            schema = parenttable.metadata.schema
+
+        tablekey = _get_table_key(tname, schema)
+        return parenttable, tablekey, colname
+
+    def _link_to_col_by_colstring(self, parenttable, table, colname):
+
+        _column = None
+        if colname is None:
+            # colname is None in the case that ForeignKey argument
+            # was specified as table name only, in which case we
+            # match the column name to the same column on the
+            # parent.
+            # this use case wasn't working in later 1.x series
+            # as it had no test coverage; fixed in 2.0
+            parent = self.parent
+            assert parent is not None
+            key = parent.key
+            _column = table.c.get(key, None)
+        elif self.link_to_name:
+            key = colname
+            for c in table.c:
+                if c.name == colname:
+                    _column = c
+        else:
+            key = colname
+            _column = table.c.get(colname, None)
+
+        if _column is None:
+            raise exc.NoReferencedColumnError(
+                "Could not initialize target column "
+                "for ForeignKey '%s' on table '%s': "
+                "table '%s' has no column named '%s'"
+                % (self._colspec, parenttable.name, table.name, key),
+                table.name,
+                key,
+            )
+
+        return _column
+
+    def _set_target_column(self, column):
+        assert self.parent is not None
+
+        # propagate TypeEngine to parent if it didn't have one
+        if self.parent.type._isnull:
+            self.parent.type = column.type
+
+        # super-edgy case, if other FKs point to our column,
+        # they'd get the type propagated out also.
+
+        def set_type(fk):
+            if fk.parent.type._isnull:
+                fk.parent.type = column.type
+
+        self.parent._setup_on_memoized_fks(set_type)
+
+        self.column = column
+
+    @util.memoized_property
+    def column(self):
+        """Return the target :class:`_schema.Column` referenced by this
+        :class:`_schema.ForeignKey`.
+
+        If no target column has been established, an exception
+        is raised.
+
+        .. versionchanged:: 0.9.0
+            Foreign key target column resolution now occurs as soon as both
+            the ForeignKey object and the remote Column to which it refers
+            are both associated with the same MetaData object.
+
+        """
+
+        return self._resolve_column()
+
+    def _resolve_column(self, raiseerr=True):
+
+        if isinstance(self._colspec, util.string_types):
+
+            parenttable, tablekey, colname = self._resolve_col_tokens()
+
+            if self._unresolvable or tablekey not in parenttable.metadata:
+                if not raiseerr:
+                    return None
+                raise exc.NoReferencedTableError(
+                    "Foreign key associated with column '%s' could not find "
+                    "table '%s' with which to generate a "
+                    "foreign key to target column '%s'"
+                    % (self.parent, tablekey, colname),
+                    tablekey,
+                )
+            elif parenttable.key not in parenttable.metadata:
+                if not raiseerr:
+                    return None
+                raise exc.InvalidRequestError(
+                    "Table %s is no longer associated with its "
+                    "parent MetaData" % parenttable
+                )
+            else:
+                table = parenttable.metadata.tables[tablekey]
+                return self._link_to_col_by_colstring(
+                    parenttable, table, colname
+                )
+
+        elif hasattr(self._colspec, "__clause_element__"):
+            _column = self._colspec.__clause_element__()
+            return _column
+        else:
+            _column = self._colspec
+            return _column
+
+    def _set_parent(self, column, **kw):
+        if self.parent is not None and self.parent is not column:
+            raise exc.InvalidRequestError(
+                "This ForeignKey already has a parent !"
+            )
+        self.parent = column
+        self.parent.foreign_keys.add(self)
+        self.parent._on_table_attach(self._set_table)
+
+    def _set_remote_table(self, table):
+        parenttable, tablekey, colname = self._resolve_col_tokens()
+        self._link_to_col_by_colstring(parenttable, table, colname)
+
+        _column = self._link_to_col_by_colstring(parenttable, table, colname)
+        self._set_target_column(_column)
+        assert self.constraint is not None
+
+        self.constraint._validate_dest_table(table)
+
+    def _remove_from_metadata(self, metadata):
+        parenttable, table_key, colname = self._resolve_col_tokens()
+        fk_key = (table_key, colname)
+
+        if self in metadata._fk_memos[fk_key]:
+            # TODO: no test coverage for self not in memos
+            metadata._fk_memos[fk_key].remove(self)
+
+    def _set_table(self, column, table):
+        # standalone ForeignKey - create ForeignKeyConstraint
+        # on the hosting Table when attached to the Table.
+        assert isinstance(table, Table)
+        if self.constraint is None:
+            self.constraint = ForeignKeyConstraint(
+                [],
+                [],
+                use_alter=self.use_alter,
+                name=self.name,
+                onupdate=self.onupdate,
+                ondelete=self.ondelete,
+                deferrable=self.deferrable,
+                initially=self.initially,
+                match=self.match,
+                **self._unvalidated_dialect_kw
+            )
+            self.constraint._append_element(column, self)
+            self.constraint._set_parent_with_dispatch(table)
+        table.foreign_keys.add(self)
+        # set up remote ".column" attribute, or a note to pick it
+        # up when the other Table/Column shows up
+        if isinstance(self._colspec, util.string_types):
+            parenttable, table_key, colname = self._resolve_col_tokens()
+            fk_key = (table_key, colname)
+            if table_key in parenttable.metadata.tables:
+                table = parenttable.metadata.tables[table_key]
+                try:
+                    _column = self._link_to_col_by_colstring(
+                        parenttable, table, colname
+                    )
+                except exc.NoReferencedColumnError:
+                    # this is OK, we'll try later
+                    pass
+                else:
+                    self._set_target_column(_column)
+            parenttable.metadata._fk_memos[fk_key].append(self)
+        elif hasattr(self._colspec, "__clause_element__"):
+            _column = self._colspec.__clause_element__()
+            self._set_target_column(_column)
+        else:
+            _column = self._colspec
+            self._set_target_column(_column)
+
+
+class DefaultGenerator(Executable, SchemaItem):
+    """Base class for column *default* values."""
+
+    __visit_name__ = "default_generator"
+
+    is_sequence = False
+    is_server_default = False
+    column = None
+
+    def __init__(self, for_update=False):
+        self.for_update = for_update
+
+    def _set_parent(self, column, **kw):
+        self.column = column
+        if self.for_update:
+            self.column.onupdate = self
+        else:
+            self.column.default = self
+
+    @util.deprecated_20(
+        ":meth:`.DefaultGenerator.execute`",
+        alternative="All statement execution in SQLAlchemy 2.0 is performed "
+        "by the :meth:`_engine.Connection.execute` method of "
+        ":class:`_engine.Connection`, "
+        "or in the ORM by the :meth:`.Session.execute` method of "
+        ":class:`.Session`.",
+    )
+    def execute(self, bind=None):
+        if bind is None:
+            bind = _bind_or_error(self)
+        return bind._execute_default(self, (), util.EMPTY_DICT)
+
+    def _execute_on_connection(
+        self, connection, multiparams, params, execution_options
+    ):
+        return connection._execute_default(
+            self, multiparams, params, execution_options
+        )
+
+    @property
+    def bind(self):
+        """Return the connectable associated with this default."""
+        if getattr(self, "column", None) is not None:
+            return self.column.table.bind
+        else:
+            return None
+
+
+class ColumnDefault(DefaultGenerator):
+    """A plain default value on a column.
+
+    This could correspond to a constant, a callable function,
+    or a SQL clause.
+
+    :class:`.ColumnDefault` is generated automatically
+    whenever the ``default``, ``onupdate`` arguments of
+    :class:`_schema.Column` are used.  A :class:`.ColumnDefault`
+    can be passed positionally as well.
+
+    For example, the following::
+
+        Column('foo', Integer, default=50)
+
+    Is equivalent to::
+
+        Column('foo', Integer, ColumnDefault(50))
+
+
+    """
+
+    def __init__(self, arg, **kwargs):
+        """Construct a new :class:`.ColumnDefault`.
+
+
+        :param arg: argument representing the default value.
+         May be one of the following:
+
+         * a plain non-callable Python value, such as a
+           string, integer, boolean, or other simple type.
+           The default value will be used as is each time.
+         * a SQL expression, that is one which derives from
+           :class:`_expression.ColumnElement`.  The SQL expression will
+           be rendered into the INSERT or UPDATE statement,
+           or in the case of a primary key column when
+           RETURNING is not used may be
+           pre-executed before an INSERT within a SELECT.
+         * A Python callable.  The function will be invoked for each
+           new row subject to an INSERT or UPDATE.
+           The callable must accept exactly
+           zero or one positional arguments.  The one-argument form
+           will receive an instance of the :class:`.ExecutionContext`,
+           which provides contextual information as to the current
+           :class:`_engine.Connection` in use as well as the current
+           statement and parameters.
+
+        """
+        super(ColumnDefault, self).__init__(**kwargs)
+        if isinstance(arg, FetchedValue):
+            raise exc.ArgumentError(
+                "ColumnDefault may not be a server-side default type."
+            )
+        if callable(arg):
+            arg = self._maybe_wrap_callable(arg)
+        self.arg = arg
+
+    @util.memoized_property
+    def is_callable(self):
+        return callable(self.arg)
+
+    @util.memoized_property
+    def is_clause_element(self):
+        return isinstance(self.arg, ClauseElement)
+
+    @util.memoized_property
+    def is_scalar(self):
+        return (
+            not self.is_callable
+            and not self.is_clause_element
+            and not self.is_sequence
+        )
+
+    @util.memoized_property
+    @util.preload_module("sqlalchemy.sql.sqltypes")
+    def _arg_is_typed(self):
+        sqltypes = util.preloaded.sql_sqltypes
+
+        if self.is_clause_element:
+            return not isinstance(self.arg.type, sqltypes.NullType)
+        else:
+            return False
+
+    def _maybe_wrap_callable(self, fn):
+        """Wrap callables that don't accept a context.
+
+        This is to allow easy compatibility with default callables
+        that aren't specific to accepting of a context.
+
+        """
+        try:
+            argspec = util.get_callable_argspec(fn, no_self=True)
+        except TypeError:
+            return util.wrap_callable(lambda ctx: fn(), fn)
+
+        defaulted = argspec[3] is not None and len(argspec[3]) or 0
+        positionals = len(argspec[0]) - defaulted
+
+        if positionals == 0:
+            return util.wrap_callable(lambda ctx: fn(), fn)
+
+        elif positionals == 1:
+            return fn
+        else:
+            raise exc.ArgumentError(
+                "ColumnDefault Python function takes zero or one "
+                "positional arguments"
+            )
+
+    def __repr__(self):
+        return "ColumnDefault(%r)" % (self.arg,)
+
+
+class IdentityOptions(object):
+    """Defines options for a named database sequence or an identity column.
+
+    .. versionadded:: 1.3.18
+
+    .. seealso::
+
+        :class:`.Sequence`
+
+    """
+
+    def __init__(
+        self,
+        start=None,
+        increment=None,
+        minvalue=None,
+        maxvalue=None,
+        nominvalue=None,
+        nomaxvalue=None,
+        cycle=None,
+        cache=None,
+        order=None,
+    ):
+        """Construct a :class:`.IdentityOptions` object.
+
+        See the :class:`.Sequence` documentation for a complete description
+        of the parameters.
+
+        :param start: the starting index of the sequence.
+        :param increment: the increment value of the sequence.
+        :param minvalue: the minimum value of the sequence.
+        :param maxvalue: the maximum value of the sequence.
+        :param nominvalue: no minimum value of the sequence.
+        :param nomaxvalue: no maximum value of the sequence.
+        :param cycle: allows the sequence to wrap around when the maxvalue
+         or minvalue has been reached.
+        :param cache: optional integer value; number of future values in the
+         sequence which are calculated in advance.
+        :param order: optional boolean value; if ``True``, renders the
+         ORDER keyword.
+
+        """
+        self.start = start
+        self.increment = increment
+        self.minvalue = minvalue
+        self.maxvalue = maxvalue
+        self.nominvalue = nominvalue
+        self.nomaxvalue = nomaxvalue
+        self.cycle = cycle
+        self.cache = cache
+        self.order = order
+
+
+class Sequence(IdentityOptions, DefaultGenerator):
+    """Represents a named database sequence.
+
+    The :class:`.Sequence` object represents the name and configurational
+    parameters of a database sequence.   It also represents
+    a construct that can be "executed" by a SQLAlchemy :class:`_engine.Engine`
+    or :class:`_engine.Connection`,
+    rendering the appropriate "next value" function
+    for the target database and returning a result.
+
+    The :class:`.Sequence` is typically associated with a primary key column::
+
+        some_table = Table(
+            'some_table', metadata,
+            Column('id', Integer, Sequence('some_table_seq'),
+            primary_key=True)
+        )
+
+    When CREATE TABLE is emitted for the above :class:`_schema.Table`, if the
+    target platform supports sequences, a CREATE SEQUENCE statement will
+    be emitted as well.   For platforms that don't support sequences,
+    the :class:`.Sequence` construct is ignored.
+
+    .. seealso::
+
+        :ref:`defaults_sequences`
+
+        :class:`.CreateSequence`
+
+        :class:`.DropSequence`
+
+    """
+
+    __visit_name__ = "sequence"
+
+    is_sequence = True
+
+    def __init__(
+        self,
+        name,
+        start=None,
+        increment=None,
+        minvalue=None,
+        maxvalue=None,
+        nominvalue=None,
+        nomaxvalue=None,
+        cycle=None,
+        schema=None,
+        cache=None,
+        order=None,
+        data_type=None,
+        optional=False,
+        quote=None,
+        metadata=None,
+        quote_schema=None,
+        for_update=False,
+    ):
+        """Construct a :class:`.Sequence` object.
+
+        :param name: the name of the sequence.
+
+        :param start: the starting index of the sequence.  This value is
+         used when the CREATE SEQUENCE command is emitted to the database
+         as the value of the "START WITH" clause.   If ``None``, the
+         clause is omitted, which on most platforms indicates a starting
+         value of 1.
+        :param increment: the increment value of the sequence.  This
+         value is used when the CREATE SEQUENCE command is emitted to
+         the database as the value of the "INCREMENT BY" clause.  If ``None``,
+         the clause is omitted, which on most platforms indicates an
+         increment of 1.
+        :param minvalue: the minimum value of the sequence.  This
+         value is used when the CREATE SEQUENCE command is emitted to
+         the database as the value of the "MINVALUE" clause.  If ``None``,
+         the clause is omitted, which on most platforms indicates a
+         minvalue of 1 and -2^63-1 for ascending and descending sequences,
+         respectively.
+
+         .. versionadded:: 1.0.7
+
+        :param maxvalue: the maximum value of the sequence.  This
+         value is used when the CREATE SEQUENCE command is emitted to
+         the database as the value of the "MAXVALUE" clause.  If ``None``,
+         the clause is omitted, which on most platforms indicates a
+         maxvalue of 2^63-1 and -1 for ascending and descending sequences,
+         respectively.
+
+         .. versionadded:: 1.0.7
+
+        :param nominvalue: no minimum value of the sequence.  This
+         value is used when the CREATE SEQUENCE command is emitted to
+         the database as the value of the "NO MINVALUE" clause.  If ``None``,
+         the clause is omitted, which on most platforms indicates a
+         minvalue of 1 and -2^63-1 for ascending and descending sequences,
+         respectively.
+
+         .. versionadded:: 1.0.7
+
+        :param nomaxvalue: no maximum value of the sequence.  This
+         value is used when the CREATE SEQUENCE command is emitted to
+         the database as the value of the "NO MAXVALUE" clause.  If ``None``,
+         the clause is omitted, which on most platforms indicates a
+         maxvalue of 2^63-1 and -1 for ascending and descending sequences,
+         respectively.
+
+         .. versionadded:: 1.0.7
+
+        :param cycle: allows the sequence to wrap around when the maxvalue
+         or minvalue has been reached by an ascending or descending sequence
+         respectively.  This value is used when the CREATE SEQUENCE command
+         is emitted to the database as the "CYCLE" clause.  If the limit is
+         reached, the next number generated will be the minvalue or maxvalue,
+         respectively.  If cycle=False (the default) any calls to nextval
+         after the sequence has reached its maximum value will return an
+         error.
+
+         .. versionadded:: 1.0.7
+
+        :param schema: optional schema name for the sequence, if located
+         in a schema other than the default.  The rules for selecting the
+         schema name when a :class:`_schema.MetaData`
+         is also present are the same
+         as that of :paramref:`_schema.Table.schema`.
+
+        :param cache: optional integer value; number of future values in the
+         sequence which are calculated in advance.  Renders the CACHE keyword
+         understood by Oracle and PostgreSQL.
+
+         .. versionadded:: 1.1.12
+
+        :param order: optional boolean value; if ``True``, renders the
+         ORDER keyword, understood by Oracle, indicating the sequence is
+         definitively ordered.   May be necessary to provide deterministic
+         ordering using Oracle RAC.
+
+         .. versionadded:: 1.1.12
+
+        :param data_type: The type to be returned by the sequence, for
+         dialects that allow us to choose between INTEGER, BIGINT, etc.
+         (e.g., mssql).
+
+         .. versionadded:: 1.4.0
+
+        :param optional: boolean value, when ``True``, indicates that this
+         :class:`.Sequence` object only needs to be explicitly generated
+         on backends that don't provide another way to generate primary
+         key identifiers.  Currently, it essentially means, "don't create
+         this sequence on the PostgreSQL backend, where the SERIAL keyword
+         creates a sequence for us automatically".
+        :param quote: boolean value, when ``True`` or ``False``, explicitly
+         forces quoting of the :paramref:`_schema.Sequence.name` on or off.
+         When left at its default of ``None``, normal quoting rules based
+         on casing and reserved words take place.
+        :param quote_schema: Set the quoting preferences for the ``schema``
+         name.
+
+        :param metadata: optional :class:`_schema.MetaData` object which this
+         :class:`.Sequence` will be associated with.  A :class:`.Sequence`
+         that is associated with a :class:`_schema.MetaData`
+         gains the following
+         capabilities:
+
+         * The :class:`.Sequence` will inherit the
+           :paramref:`_schema.MetaData.schema`
+           parameter specified to the target :class:`_schema.MetaData`, which
+           affects the production of CREATE / DROP DDL, if any.
+
+         * The :meth:`.Sequence.create` and :meth:`.Sequence.drop` methods
+           automatically use the engine bound to the :class:`_schema.MetaData`
+           object, if any.
+
+         * The :meth:`_schema.MetaData.create_all` and
+           :meth:`_schema.MetaData.drop_all`
+           methods will emit CREATE / DROP for this :class:`.Sequence`,
+           even if the :class:`.Sequence` is not associated with any
+           :class:`_schema.Table` / :class:`_schema.Column`
+           that's a member of this
+           :class:`_schema.MetaData`.
+
+         The above behaviors can only occur if the :class:`.Sequence` is
+         explicitly associated with the :class:`_schema.MetaData`
+         via this parameter.
+
+         .. seealso::
+
+            :ref:`sequence_metadata` - full discussion of the
+            :paramref:`.Sequence.metadata` parameter.
+
+        :param for_update: Indicates this :class:`.Sequence`, when associated
+         with a :class:`_schema.Column`,
+         should be invoked for UPDATE statements
+         on that column's table, rather than for INSERT statements, when
+         no value is otherwise present for that column in the statement.
+
+        """
+        DefaultGenerator.__init__(self, for_update=for_update)
+        IdentityOptions.__init__(
+            self,
+            start=start,
+            increment=increment,
+            minvalue=minvalue,
+            maxvalue=maxvalue,
+            nominvalue=nominvalue,
+            nomaxvalue=nomaxvalue,
+            cycle=cycle,
+            cache=cache,
+            order=order,
+        )
+        self.name = quoted_name(name, quote)
+        self.optional = optional
+        if schema is BLANK_SCHEMA:
+            self.schema = schema = None
+        elif metadata is not None and schema is None and metadata.schema:
+            self.schema = schema = metadata.schema
+        else:
+            self.schema = quoted_name(schema, quote_schema)
+        self.metadata = metadata
+        self._key = _get_table_key(name, schema)
+        if metadata:
+            self._set_metadata(metadata)
+        if data_type is not None:
+            self.data_type = to_instance(data_type)
+        else:
+            self.data_type = None
+
+    @util.memoized_property
+    def is_callable(self):
+        return False
+
+    @util.memoized_property
+    def is_clause_element(self):
+        return False
+
+    @util.preload_module("sqlalchemy.sql.functions")
+    def next_value(self):
+        """Return a :class:`.next_value` function element
+        which will render the appropriate increment function
+        for this :class:`.Sequence` within any SQL expression.
+
+        """
+        if self.bind:
+            return util.preloaded.sql_functions.func.next_value(
+                self, bind=self.bind
+            )
+        else:
+            return util.preloaded.sql_functions.func.next_value(self)
+
+    def _set_parent(self, column, **kw):
+        super(Sequence, self)._set_parent(column)
+        column._on_table_attach(self._set_table)
+
+    def _set_table(self, column, table):
+        self._set_metadata(table.metadata)
+
+    def _set_metadata(self, metadata):
+        self.metadata = metadata
+        self.metadata._sequences[self._key] = self
+
+    @property
+    def bind(self):
+        if self.metadata:
+            return self.metadata.bind
+        else:
+            return None
+
+    def create(self, bind=None, checkfirst=True):
+        """Creates this sequence in the database.
+
+        .. note:: the "bind" argument will be required in
+           SQLAlchemy 2.0.
+
+        """
+
+        if bind is None:
+            bind = _bind_or_error(self)
+        bind._run_ddl_visitor(ddl.SchemaGenerator, self, checkfirst=checkfirst)
+
+    def drop(self, bind=None, checkfirst=True):
+        """Drops this sequence from the database.
+
+        .. note:: the "bind" argument will be required in
+           SQLAlchemy 2.0.
+
+        """
+
+        if bind is None:
+            bind = _bind_or_error(self)
+        bind._run_ddl_visitor(ddl.SchemaDropper, self, checkfirst=checkfirst)
+
+    def _not_a_column_expr(self):
+        raise exc.InvalidRequestError(
+            "This %s cannot be used directly "
+            "as a column expression.  Use func.next_value(sequence) "
+            "to produce a 'next value' function that's usable "
+            "as a column element." % self.__class__.__name__
+        )
+
+
+@inspection._self_inspects
+class FetchedValue(SchemaEventTarget):
+    """A marker for a transparent database-side default.
+
+    Use :class:`.FetchedValue` when the database is configured
+    to provide some automatic default for a column.
+
+    E.g.::
+
+        Column('foo', Integer, FetchedValue())
+
+    Would indicate that some trigger or default generator
+    will create a new value for the ``foo`` column during an
+    INSERT.
+
+    .. seealso::
+
+        :ref:`triggered_columns`
+
+    """
+
+    is_server_default = True
+    reflected = False
+    has_argument = False
+    is_clause_element = False
+
+    def __init__(self, for_update=False):
+        self.for_update = for_update
+
+    def _as_for_update(self, for_update):
+        if for_update == self.for_update:
+            return self
+        else:
+            return self._clone(for_update)
+
+    def _clone(self, for_update):
+        n = self.__class__.__new__(self.__class__)
+        n.__dict__.update(self.__dict__)
+        n.__dict__.pop("column", None)
+        n.for_update = for_update
+        return n
+
+    def _set_parent(self, column, **kw):
+        self.column = column
+        if self.for_update:
+            self.column.server_onupdate = self
+        else:
+            self.column.server_default = self
+
+    def __repr__(self):
+        return util.generic_repr(self)
+
+
+class DefaultClause(FetchedValue):
+    """A DDL-specified DEFAULT column value.
+
+    :class:`.DefaultClause` is a :class:`.FetchedValue`
+    that also generates a "DEFAULT" clause when
+    "CREATE TABLE" is emitted.
+
+    :class:`.DefaultClause` is generated automatically
+    whenever the ``server_default``, ``server_onupdate`` arguments of
+    :class:`_schema.Column` are used.  A :class:`.DefaultClause`
+    can be passed positionally as well.
+
+    For example, the following::
+
+        Column('foo', Integer, server_default="50")
+
+    Is equivalent to::
+
+        Column('foo', Integer, DefaultClause("50"))
+
+    """
+
+    has_argument = True
+
+    def __init__(self, arg, for_update=False, _reflected=False):
+        util.assert_arg_type(
+            arg, (util.string_types[0], ClauseElement, TextClause), "arg"
+        )
+        super(DefaultClause, self).__init__(for_update)
+        self.arg = arg
+        self.reflected = _reflected
+
+    def __repr__(self):
+        return "DefaultClause(%r, for_update=%r)" % (self.arg, self.for_update)
+
+
+class Constraint(DialectKWArgs, SchemaItem):
+    """A table-level SQL constraint.
+
+    :class:`_schema.Constraint` serves as the base class for the series of
+    constraint objects that can be associated with :class:`_schema.Table`
+    objects, including :class:`_schema.PrimaryKeyConstraint`,
+    :class:`_schema.ForeignKeyConstraint`
+    :class:`_schema.UniqueConstraint`, and
+    :class:`_schema.CheckConstraint`.
+
+    """
+
+    __visit_name__ = "constraint"
+
+    def __init__(
+        self,
+        name=None,
+        deferrable=None,
+        initially=None,
+        _create_rule=None,
+        info=None,
+        _type_bound=False,
+        **dialect_kw
+    ):
+        r"""Create a SQL constraint.
+
+        :param name:
+          Optional, the in-database name of this ``Constraint``.
+
+        :param deferrable:
+          Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
+          issuing DDL for this constraint.
+
+        :param initially:
+          Optional string.  If set, emit INITIALLY <value> when issuing DDL
+          for this constraint.
+
+        :param info: Optional data dictionary which will be populated into the
+            :attr:`.SchemaItem.info` attribute of this object.
+
+            .. versionadded:: 1.0.0
+
+        :param \**dialect_kw:  Additional keyword arguments are dialect
+            specific, and passed in the form ``<dialectname>_<argname>``.  See
+            the documentation regarding an individual dialect at
+            :ref:`dialect_toplevel` for detail on documented arguments.
+
+        :param _create_rule:
+          used internally by some datatypes that also create constraints.
+
+        :param _type_bound:
+          used internally to indicate that this constraint is associated with
+          a specific datatype.
+
+        """
+
+        self.name = name
+        self.deferrable = deferrable
+        self.initially = initially
+        if info:
+            self.info = info
+        self._create_rule = _create_rule
+        self._type_bound = _type_bound
+        util.set_creation_order(self)
+        self._validate_dialect_kwargs(dialect_kw)
+
+    @property
+    def table(self):
+        try:
+            if isinstance(self.parent, Table):
+                return self.parent
+        except AttributeError:
+            pass
+        raise exc.InvalidRequestError(
+            "This constraint is not bound to a table.  Did you "
+            "mean to call table.append_constraint(constraint) ?"
+        )
+
+    def _set_parent(self, parent, **kw):
+        self.parent = parent
+        parent.constraints.add(self)
+
+    @util.deprecated(
+        "1.4",
+        "The :meth:`_schema.Constraint.copy` method is deprecated "
+        "and will be removed in a future release.",
+    )
+    def copy(self, **kw):
+        return self._copy(**kw)
+
+    def _copy(self, **kw):
+        raise NotImplementedError()
+
+
+class ColumnCollectionMixin(object):
+
+    columns = None
+    """A :class:`_expression.ColumnCollection` of :class:`_schema.Column`
+    objects.
+
+    This collection represents the columns which are referred to by
+    this object.
+
+    """
+
+    _allow_multiple_tables = False
+
+    def __init__(self, *columns, **kw):
+        _autoattach = kw.pop("_autoattach", True)
+        self._column_flag = kw.pop("_column_flag", False)
+        self.columns = DedupeColumnCollection()
+
+        processed_expressions = kw.pop("_gather_expressions", None)
+        if processed_expressions is not None:
+            self._pending_colargs = []
+            for (
+                expr,
+                column,
+                strname,
+                add_element,
+            ) in coercions.expect_col_expression_collection(
+                roles.DDLConstraintColumnRole, columns
+            ):
+                self._pending_colargs.append(add_element)
+                processed_expressions.append(expr)
+        else:
+            self._pending_colargs = [
+                coercions.expect(roles.DDLConstraintColumnRole, column)
+                for column in columns
+            ]
+
+        if _autoattach and self._pending_colargs:
+            self._check_attach()
+
+    def _check_attach(self, evt=False):
+        col_objs = [c for c in self._pending_colargs if isinstance(c, Column)]
+
+        cols_w_table = [c for c in col_objs if isinstance(c.table, Table)]
+
+        cols_wo_table = set(col_objs).difference(cols_w_table)
+        if cols_wo_table:
+            # feature #3341 - place event listeners for Column objects
+            # such that when all those cols are attached, we autoattach.
+            assert not evt, "Should not reach here on event call"
+
+            # issue #3411 - don't do the per-column auto-attach if some of the
+            # columns are specified as strings.
+            has_string_cols = set(
+                c for c in self._pending_colargs if c is not None
+            ).difference(col_objs)
+            if not has_string_cols:
+
+                def _col_attached(column, table):
+                    # this isinstance() corresponds with the
+                    # isinstance() above; only want to count Table-bound
+                    # columns
+                    if isinstance(table, Table):
+                        cols_wo_table.discard(column)
+                        if not cols_wo_table:
+                            self._check_attach(evt=True)
+
+                self._cols_wo_table = cols_wo_table
+                for col in cols_wo_table:
+                    col._on_table_attach(_col_attached)
+                return
+
+        columns = cols_w_table
+
+        tables = {c.table for c in columns}
+        if len(tables) == 1:
+            self._set_parent_with_dispatch(tables.pop())
+        elif len(tables) > 1 and not self._allow_multiple_tables:
+            table = columns[0].table
+            others = [c for c in columns[1:] if c.table is not table]
+            if others:
+                raise exc.ArgumentError(
+                    "Column(s) %s are not part of table '%s'."
+                    % (
+                        ", ".join("'%s'" % c for c in others),
+                        table.description,
+                    )
+                )
+
+    def _col_expressions(self, table):
+        return [
+            table.c[col] if isinstance(col, util.string_types) else col
+            for col in self._pending_colargs
+        ]
+
+    def _set_parent(self, table, **kw):
+        for col in self._col_expressions(table):
+            if col is not None:
+                self.columns.add(col)
+
+
+class ColumnCollectionConstraint(ColumnCollectionMixin, Constraint):
+    """A constraint that proxies a ColumnCollection."""
+
+    def __init__(self, *columns, **kw):
+        r"""
+        :param \*columns:
+          A sequence of column names or Column objects.
+
+        :param name:
+          Optional, the in-database name of this constraint.
+
+        :param deferrable:
+          Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
+          issuing DDL for this constraint.
+
+        :param initially:
+          Optional string.  If set, emit INITIALLY <value> when issuing DDL
+          for this constraint.
+
+        :param \**kw: other keyword arguments including dialect-specific
+          arguments are propagated to the :class:`.Constraint` superclass.
+
+        """
+        _autoattach = kw.pop("_autoattach", True)
+        _column_flag = kw.pop("_column_flag", False)
+        Constraint.__init__(self, **kw)
+        ColumnCollectionMixin.__init__(
+            self, *columns, _autoattach=_autoattach, _column_flag=_column_flag
+        )
+
+    columns = None
+    """A :class:`_expression.ColumnCollection` representing the set of columns
+    for this constraint.
+
+    """
+
+    def _set_parent(self, table, **kw):
+        Constraint._set_parent(self, table)
+        ColumnCollectionMixin._set_parent(self, table)
+
+    def __contains__(self, x):
+        return x in self.columns
+
+    @util.deprecated(
+        "1.4",
+        "The :meth:`_schema.ColumnCollectionConstraint.copy` method "
+        "is deprecated and will be removed in a future release.",
+    )
+    def copy(self, target_table=None, **kw):
+        return self._copy(target_table=target_table, **kw)
+
+    def _copy(self, target_table=None, **kw):
+        # ticket #5276
+        constraint_kwargs = {}
+        for dialect_name in self.dialect_options:
+            dialect_options = self.dialect_options[dialect_name]._non_defaults
+            for (
+                dialect_option_key,
+                dialect_option_value,
+            ) in dialect_options.items():
+                constraint_kwargs[
+                    dialect_name + "_" + dialect_option_key
+                ] = dialect_option_value
+
+        c = self.__class__(
+            name=self.name,
+            deferrable=self.deferrable,
+            initially=self.initially,
+            *[
+                _copy_expression(expr, self.parent, target_table)
+                for expr in self.columns
+            ],
+            **constraint_kwargs
+        )
+        return self._schema_item_copy(c)
+
+    def contains_column(self, col):
+        """Return True if this constraint contains the given column.
+
+        Note that this object also contains an attribute ``.columns``
+        which is a :class:`_expression.ColumnCollection` of
+        :class:`_schema.Column` objects.
+
+        """
+
+        return self.columns.contains_column(col)
+
+    def __iter__(self):
+        return iter(self.columns)
+
+    def __len__(self):
+        return len(self.columns)
+
+
+class CheckConstraint(ColumnCollectionConstraint):
+    """A table- or column-level CHECK constraint.
+
+    Can be included in the definition of a Table or Column.
+    """
+
+    _allow_multiple_tables = True
+
+    __visit_name__ = "table_or_column_check_constraint"
+
+    @_document_text_coercion(
+        "sqltext",
+        ":class:`.CheckConstraint`",
+        ":paramref:`.CheckConstraint.sqltext`",
+    )
+    def __init__(
+        self,
+        sqltext,
+        name=None,
+        deferrable=None,
+        initially=None,
+        table=None,
+        info=None,
+        _create_rule=None,
+        _autoattach=True,
+        _type_bound=False,
+        **kw
+    ):
+        r"""Construct a CHECK constraint.
+
+        :param sqltext:
+         A string containing the constraint definition, which will be used
+         verbatim, or a SQL expression construct.   If given as a string,
+         the object is converted to a :func:`_expression.text` object.
+         If the textual
+         string includes a colon character, escape this using a backslash::
+
+           CheckConstraint(r"foo ~ E'a(?\:b|c)d")
+
+        :param name:
+          Optional, the in-database name of the constraint.
+
+        :param deferrable:
+          Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
+          issuing DDL for this constraint.
+
+        :param initially:
+          Optional string.  If set, emit INITIALLY <value> when issuing DDL
+          for this constraint.
+
+        :param info: Optional data dictionary which will be populated into the
+            :attr:`.SchemaItem.info` attribute of this object.
+
+            .. versionadded:: 1.0.0
+
+        """
+
+        self.sqltext = coercions.expect(roles.DDLExpressionRole, sqltext)
+        columns = []
+        visitors.traverse(self.sqltext, {}, {"column": columns.append})
+
+        super(CheckConstraint, self).__init__(
+            name=name,
+            deferrable=deferrable,
+            initially=initially,
+            _create_rule=_create_rule,
+            info=info,
+            _type_bound=_type_bound,
+            _autoattach=_autoattach,
+            *columns,
+            **kw
+        )
+        if table is not None:
+            self._set_parent_with_dispatch(table)
+
+    @property
+    def is_column_level(self):
+        return not isinstance(self.parent, Table)
+
+    @util.deprecated(
+        "1.4",
+        "The :meth:`_schema.CheckConstraint.copy` method is deprecated "
+        "and will be removed in a future release.",
+    )
+    def copy(self, target_table=None, **kw):
+        return self._copy(target_table=target_table, **kw)
+
+    def _copy(self, target_table=None, **kw):
+        if target_table is not None:
+            # note that target_table is None for the copy process of
+            # a column-bound CheckConstraint, so this path is not reached
+            # in that case.
+            sqltext = _copy_expression(self.sqltext, self.table, target_table)
+        else:
+            sqltext = self.sqltext
+        c = CheckConstraint(
+            sqltext,
+            name=self.name,
+            initially=self.initially,
+            deferrable=self.deferrable,
+            _create_rule=self._create_rule,
+            table=target_table,
+            _autoattach=False,
+            _type_bound=self._type_bound,
+        )
+        return self._schema_item_copy(c)
+
+
+class ForeignKeyConstraint(ColumnCollectionConstraint):
+    """A table-level FOREIGN KEY constraint.
+
+    Defines a single column or composite FOREIGN KEY ... REFERENCES
+    constraint. For a no-frills, single column foreign key, adding a
+    :class:`_schema.ForeignKey` to the definition of a :class:`_schema.Column`
+    is a
+    shorthand equivalent for an unnamed, single column
+    :class:`_schema.ForeignKeyConstraint`.
+
+    Examples of foreign key configuration are in :ref:`metadata_foreignkeys`.
+
+    """
+
+    __visit_name__ = "foreign_key_constraint"
+
+    def __init__(
+        self,
+        columns,
+        refcolumns,
+        name=None,
+        onupdate=None,
+        ondelete=None,
+        deferrable=None,
+        initially=None,
+        use_alter=False,
+        link_to_name=False,
+        match=None,
+        table=None,
+        info=None,
+        **dialect_kw
+    ):
+        r"""Construct a composite-capable FOREIGN KEY.
+
+        :param columns: A sequence of local column names. The named columns
+          must be defined and present in the parent Table. The names should
+          match the ``key`` given to each column (defaults to the name) unless
+          ``link_to_name`` is True.
+
+        :param refcolumns: A sequence of foreign column names or Column
+          objects. The columns must all be located within the same Table.
+
+        :param name: Optional, the in-database name of the key.
+
+        :param onupdate: Optional string. If set, emit ON UPDATE <value> when
+          issuing DDL for this constraint. Typical values include CASCADE,
+          DELETE and RESTRICT.
+
+        :param ondelete: Optional string. If set, emit ON DELETE <value> when
+          issuing DDL for this constraint. Typical values include CASCADE,
+          DELETE and RESTRICT.
+
+        :param deferrable: Optional bool. If set, emit DEFERRABLE or NOT
+          DEFERRABLE when issuing DDL for this constraint.
+
+        :param initially: Optional string. If set, emit INITIALLY <value> when
+          issuing DDL for this constraint.
+
+        :param link_to_name: if True, the string name given in ``column`` is
+          the rendered name of the referenced column, not its locally assigned
+          ``key``.
+
+        :param use_alter: If True, do not emit the DDL for this constraint as
+          part of the CREATE TABLE definition. Instead, generate it via an
+          ALTER TABLE statement issued after the full collection of tables
+          have been created, and drop it via an ALTER TABLE statement before
+          the full collection of tables are dropped.
+
+          The use of :paramref:`_schema.ForeignKeyConstraint.use_alter` is
+          particularly geared towards the case where two or more tables
+          are established within a mutually-dependent foreign key constraint
+          relationship; however, the :meth:`_schema.MetaData.create_all` and
+          :meth:`_schema.MetaData.drop_all`
+          methods will perform this resolution
+          automatically, so the flag is normally not needed.
+
+          .. versionchanged:: 1.0.0  Automatic resolution of foreign key
+             cycles has been added, removing the need to use the
+             :paramref:`_schema.ForeignKeyConstraint.use_alter` in typical use
+             cases.
+
+          .. seealso::
+
+                :ref:`use_alter`
+
+        :param match: Optional string. If set, emit MATCH <value> when issuing
+          DDL for this constraint. Typical values include SIMPLE, PARTIAL
+          and FULL.
+
+        :param info: Optional data dictionary which will be populated into the
+            :attr:`.SchemaItem.info` attribute of this object.
+
+            .. versionadded:: 1.0.0
+
+        :param \**dialect_kw:  Additional keyword arguments are dialect
+          specific, and passed in the form ``<dialectname>_<argname>``.  See
+          the documentation regarding an individual dialect at
+          :ref:`dialect_toplevel` for detail on documented arguments.
+
+            .. versionadded:: 0.9.2
+
+        """
+
+        Constraint.__init__(
+            self,
+            name=name,
+            deferrable=deferrable,
+            initially=initially,
+            info=info,
+            **dialect_kw
+        )
+        self.onupdate = onupdate
+        self.ondelete = ondelete
+        self.link_to_name = link_to_name
+        self.use_alter = use_alter
+        self.match = match
+
+        if len(set(columns)) != len(refcolumns):
+            if len(set(columns)) != len(columns):
+                # e.g. FOREIGN KEY (a, a) REFERENCES r (b, c)
+                raise exc.ArgumentError(
+                    "ForeignKeyConstraint with duplicate source column "
+                    "references are not supported."
+                )
+            else:
+                # e.g. FOREIGN KEY (a) REFERENCES r (b, c)
+                # paraphrasing
+                # https://www.postgresql.org/docs/current/static/ddl-constraints.html
+                raise exc.ArgumentError(
+                    "ForeignKeyConstraint number "
+                    "of constrained columns must match the number of "
+                    "referenced columns."
+                )
+
+        # standalone ForeignKeyConstraint - create
+        # associated ForeignKey objects which will be applied to hosted
+        # Column objects (in col.foreign_keys), either now or when attached
+        # to the Table for string-specified names
+        self.elements = [
+            ForeignKey(
+                refcol,
+                _constraint=self,
+                name=self.name,
+                onupdate=self.onupdate,
+                ondelete=self.ondelete,
+                use_alter=self.use_alter,
+                link_to_name=self.link_to_name,
+                match=self.match,
+                deferrable=self.deferrable,
+                initially=self.initially,
+                **self.dialect_kwargs
+            )
+            for refcol in refcolumns
+        ]
+
+        ColumnCollectionMixin.__init__(self, *columns)
+        if table is not None:
+            if hasattr(self, "parent"):
+                assert table is self.parent
+            self._set_parent_with_dispatch(table)
+
+    def _append_element(self, column, fk):
+        self.columns.add(column)
+        self.elements.append(fk)
+
+    columns = None
+    """A :class:`_expression.ColumnCollection` representing the set of columns
+    for this constraint.
+
+    """
+
+    elements = None
+    """A sequence of :class:`_schema.ForeignKey` objects.
+
+    Each :class:`_schema.ForeignKey`
+    represents a single referring column/referred
+    column pair.
+
+    This collection is intended to be read-only.
+
+    """
+
+    @property
+    def _elements(self):
+        # legacy - provide a dictionary view of (column_key, fk)
+        return util.OrderedDict(zip(self.column_keys, self.elements))
+
+    @property
+    def _referred_schema(self):
+        for elem in self.elements:
+            return elem._referred_schema
+        else:
+            return None
+
+    @property
+    def referred_table(self):
+        """The :class:`_schema.Table` object to which this
+        :class:`_schema.ForeignKeyConstraint` references.
+
+        This is a dynamically calculated attribute which may not be available
+        if the constraint and/or parent table is not yet associated with
+        a metadata collection that contains the referred table.
+
+        .. versionadded:: 1.0.0
+
+        """
+        return self.elements[0].column.table
+
+    def _validate_dest_table(self, table):
+        table_keys = set([elem._table_key() for elem in self.elements])
+        if None not in table_keys and len(table_keys) > 1:
+            elem0, elem1 = sorted(table_keys)[0:2]
+            raise exc.ArgumentError(
+                "ForeignKeyConstraint on %s(%s) refers to "
+                "multiple remote tables: %s and %s"
+                % (table.fullname, self._col_description, elem0, elem1)
+            )
+
+    @property
+    def column_keys(self):
+        """Return a list of string keys representing the local
+        columns in this :class:`_schema.ForeignKeyConstraint`.
+
+        This list is either the original string arguments sent
+        to the constructor of the :class:`_schema.ForeignKeyConstraint`,
+        or if the constraint has been initialized with :class:`_schema.Column`
+        objects, is the string ``.key`` of each element.
+
+        .. versionadded:: 1.0.0
+
+        """
+        if hasattr(self, "parent"):
+            return self.columns.keys()
+        else:
+            return [
+                col.key if isinstance(col, ColumnElement) else str(col)
+                for col in self._pending_colargs
+            ]
+
+    @property
+    def _col_description(self):
+        return ", ".join(self.column_keys)
+
+    def _set_parent(self, table, **kw):
+        Constraint._set_parent(self, table)
+
+        try:
+            ColumnCollectionConstraint._set_parent(self, table)
+        except KeyError as ke:
+            util.raise_(
+                exc.ArgumentError(
+                    "Can't create ForeignKeyConstraint "
+                    "on table '%s': no column "
+                    "named '%s' is present." % (table.description, ke.args[0])
+                ),
+                from_=ke,
+            )
+
+        for col, fk in zip(self.columns, self.elements):
+            if not hasattr(fk, "parent") or fk.parent is not col:
+                fk._set_parent_with_dispatch(col)
+
+        self._validate_dest_table(table)
+
+    @util.deprecated(
+        "1.4",
+        "The :meth:`_schema.ForeignKeyConstraint.copy` method is deprecated "
+        "and will be removed in a future release.",
+    )
+    def copy(self, schema=None, target_table=None, **kw):
+        return self._copy(schema=schema, target_table=target_table, **kw)
+
+    def _copy(self, schema=None, target_table=None, **kw):
+        fkc = ForeignKeyConstraint(
+            [x.parent.key for x in self.elements],
+            [
+                x._get_colspec(
+                    schema=schema,
+                    table_name=target_table.name
+                    if target_table is not None
+                    and x._table_key() == x.parent.table.key
+                    else None,
+                )
+                for x in self.elements
+            ],
+            name=self.name,
+            onupdate=self.onupdate,
+            ondelete=self.ondelete,
+            use_alter=self.use_alter,
+            deferrable=self.deferrable,
+            initially=self.initially,
+            link_to_name=self.link_to_name,
+            match=self.match,
+        )
+        for self_fk, other_fk in zip(self.elements, fkc.elements):
+            self_fk._schema_item_copy(other_fk)
+        return self._schema_item_copy(fkc)
+
+
+class PrimaryKeyConstraint(ColumnCollectionConstraint):
+    """A table-level PRIMARY KEY constraint.
+
+    The :class:`.PrimaryKeyConstraint` object is present automatically
+    on any :class:`_schema.Table` object; it is assigned a set of
+    :class:`_schema.Column` objects corresponding to those marked with
+    the :paramref:`_schema.Column.primary_key` flag::
+
+        >>> my_table = Table('mytable', metadata,
+        ...                 Column('id', Integer, primary_key=True),
+        ...                 Column('version_id', Integer, primary_key=True),
+        ...                 Column('data', String(50))
+        ...     )
+        >>> my_table.primary_key
+        PrimaryKeyConstraint(
+            Column('id', Integer(), table=<mytable>,
+                   primary_key=True, nullable=False),
+            Column('version_id', Integer(), table=<mytable>,
+                   primary_key=True, nullable=False)
+        )
+
+    The primary key of a :class:`_schema.Table` can also be specified by using
+    a :class:`.PrimaryKeyConstraint` object explicitly; in this mode of usage,
+    the "name" of the constraint can also be specified, as well as other
+    options which may be recognized by dialects::
+
+        my_table = Table('mytable', metadata,
+                    Column('id', Integer),
+                    Column('version_id', Integer),
+                    Column('data', String(50)),
+                    PrimaryKeyConstraint('id', 'version_id',
+                                         name='mytable_pk')
+                )
+
+    The two styles of column-specification should generally not be mixed.
+    An warning is emitted if the columns present in the
+    :class:`.PrimaryKeyConstraint`
+    don't match the columns that were marked as ``primary_key=True``, if both
+    are present; in this case, the columns are taken strictly from the
+    :class:`.PrimaryKeyConstraint` declaration, and those columns otherwise
+    marked as ``primary_key=True`` are ignored.  This behavior is intended to
+    be backwards compatible with previous behavior.
+
+    .. versionchanged:: 0.9.2  Using a mixture of columns within a
+       :class:`.PrimaryKeyConstraint` in addition to columns marked as
+       ``primary_key=True`` now emits a warning if the lists don't match.
+       The ultimate behavior of ignoring those columns marked with the flag
+       only is currently maintained for backwards compatibility; this warning
+       may raise an exception in a future release.
+
+    For the use case where specific options are to be specified on the
+    :class:`.PrimaryKeyConstraint`, but the usual style of using
+    ``primary_key=True`` flags is still desirable, an empty
+    :class:`.PrimaryKeyConstraint` may be specified, which will take on the
+    primary key column collection from the :class:`_schema.Table` based on the
+    flags::
+
+        my_table = Table('mytable', metadata,
+                    Column('id', Integer, primary_key=True),
+                    Column('version_id', Integer, primary_key=True),
+                    Column('data', String(50)),
+                    PrimaryKeyConstraint(name='mytable_pk',
+                                         mssql_clustered=True)
+                )
+
+    .. versionadded:: 0.9.2 an empty :class:`.PrimaryKeyConstraint` may now
+       be specified for the purposes of establishing keyword arguments with
+       the constraint, independently of the specification of "primary key"
+       columns within the :class:`_schema.Table` itself; columns marked as
+       ``primary_key=True`` will be gathered into the empty constraint's
+       column collection.
+
+    """
+
+    __visit_name__ = "primary_key_constraint"
+
+    def __init__(self, *columns, **kw):
+        self._implicit_generated = kw.pop("_implicit_generated", False)
+        super(PrimaryKeyConstraint, self).__init__(*columns, **kw)
+
+    def _set_parent(self, table, **kw):
+        super(PrimaryKeyConstraint, self)._set_parent(table)
+
+        if table.primary_key is not self:
+            table.constraints.discard(table.primary_key)
+            table.primary_key = self
+            table.constraints.add(self)
+
+        table_pks = [c for c in table.c if c.primary_key]
+        if self.columns and table_pks and set(table_pks) != set(self.columns):
+            util.warn(
+                "Table '%s' specifies columns %s as primary_key=True, "
+                "not matching locally specified columns %s; setting the "
+                "current primary key columns to %s. This warning "
+                "may become an exception in a future release"
+                % (
+                    table.name,
+                    ", ".join("'%s'" % c.name for c in table_pks),
+                    ", ".join("'%s'" % c.name for c in self.columns),
+                    ", ".join("'%s'" % c.name for c in self.columns),
+                )
+            )
+            table_pks[:] = []
+
+        for c in self.columns:
+            c.primary_key = True
+            if c._user_defined_nullable is NULL_UNSPECIFIED:
+                c.nullable = False
+        if table_pks:
+            self.columns.extend(table_pks)
+
+    def _reload(self, columns):
+        """repopulate this :class:`.PrimaryKeyConstraint` given
+        a set of columns.
+
+        Existing columns in the table that are marked as primary_key=True
+        are maintained.
+
+        Also fires a new event.
+
+        This is basically like putting a whole new
+        :class:`.PrimaryKeyConstraint` object on the parent
+        :class:`_schema.Table` object without actually replacing the object.
+
+        The ordering of the given list of columns is also maintained; these
+        columns will be appended to the list of columns after any which
+        are already present.
+
+        """
+        # set the primary key flag on new columns.
+        # note any existing PK cols on the table also have their
+        # flag still set.
+        for col in columns:
+            col.primary_key = True
+
+        self.columns.extend(columns)
+
+        PrimaryKeyConstraint._autoincrement_column._reset(self)
+        self._set_parent_with_dispatch(self.table)
+
+    def _replace(self, col):
+        PrimaryKeyConstraint._autoincrement_column._reset(self)
+        self.columns.replace(col)
+
+        self.dispatch._sa_event_column_added_to_pk_constraint(self, col)
+
+    @property
+    def columns_autoinc_first(self):
+        autoinc = self._autoincrement_column
+
+        if autoinc is not None:
+            return [autoinc] + [c for c in self.columns if c is not autoinc]
+        else:
+            return list(self.columns)
+
+    @util.memoized_property
+    def _autoincrement_column(self):
+        def _validate_autoinc(col, autoinc_true):
+            if col.type._type_affinity is None or not issubclass(
+                col.type._type_affinity,
+                (
+                    type_api.INTEGERTYPE._type_affinity,
+                    type_api.NUMERICTYPE._type_affinity,
+                ),
+            ):
+                if autoinc_true:
+                    raise exc.ArgumentError(
+                        "Column type %s on column '%s' is not "
+                        "compatible with autoincrement=True" % (col.type, col)
+                    )
+                else:
+                    return False
+            elif (
+                not isinstance(col.default, (type(None), Sequence))
+                and not autoinc_true
+            ):
+                return False
+            elif (
+                col.server_default is not None
+                and not isinstance(col.server_default, Identity)
+                and not autoinc_true
+            ):
+                return False
+            elif col.foreign_keys and col.autoincrement not in (
+                True,
+                "ignore_fk",
+            ):
+                return False
+            return True
+
+        if len(self.columns) == 1:
+            col = list(self.columns)[0]
+
+            if col.autoincrement is True:
+                _validate_autoinc(col, True)
+                return col
+            elif (
+                col.autoincrement
+                in (
+                    "auto",
+                    "ignore_fk",
+                )
+                and _validate_autoinc(col, False)
+            ):
+                return col
+
+        else:
+            autoinc = None
+            for col in self.columns:
+                if col.autoincrement is True:
+                    _validate_autoinc(col, True)
+                    if autoinc is not None:
+                        raise exc.ArgumentError(
+                            "Only one Column may be marked "
+                            "autoincrement=True, found both %s and %s."
+                            % (col.name, autoinc.name)
+                        )
+                    else:
+                        autoinc = col
+
+            return autoinc
+
+
+class UniqueConstraint(ColumnCollectionConstraint):
+    """A table-level UNIQUE constraint.
+
+    Defines a single column or composite UNIQUE constraint. For a no-frills,
+    single column constraint, adding ``unique=True`` to the ``Column``
+    definition is a shorthand equivalent for an unnamed, single column
+    UniqueConstraint.
+    """
+
+    __visit_name__ = "unique_constraint"
+
+
+class Index(DialectKWArgs, ColumnCollectionMixin, SchemaItem):
+    """A table-level INDEX.
+
+    Defines a composite (one or more column) INDEX.
+
+    E.g.::
+
+        sometable = Table("sometable", metadata,
+                        Column("name", String(50)),
+                        Column("address", String(100))
+                    )
+
+        Index("some_index", sometable.c.name)
+
+    For a no-frills, single column index, adding
+    :class:`_schema.Column` also supports ``index=True``::
+
+        sometable = Table("sometable", metadata,
+                        Column("name", String(50), index=True)
+                    )
+
+    For a composite index, multiple columns can be specified::
+
+        Index("some_index", sometable.c.name, sometable.c.address)
+
+    Functional indexes are supported as well, typically by using the
+    :data:`.func` construct in conjunction with table-bound
+    :class:`_schema.Column` objects::
+
+        Index("some_index", func.lower(sometable.c.name))
+
+    An :class:`.Index` can also be manually associated with a
+    :class:`_schema.Table`,
+    either through inline declaration or using
+    :meth:`_schema.Table.append_constraint`.  When this approach is used,
+    the names
+    of the indexed columns can be specified as strings::
+
+        Table("sometable", metadata,
+                        Column("name", String(50)),
+                        Column("address", String(100)),
+                        Index("some_index", "name", "address")
+                )
+
+    To support functional or expression-based indexes in this form, the
+    :func:`_expression.text` construct may be used::
+
+        from sqlalchemy import text
+
+        Table("sometable", metadata,
+                        Column("name", String(50)),
+                        Column("address", String(100)),
+                        Index("some_index", text("lower(name)"))
+                )
+
+    .. versionadded:: 0.9.5 the :func:`_expression.text`
+       construct may be used to
+       specify :class:`.Index` expressions, provided the :class:`.Index`
+       is explicitly associated with the :class:`_schema.Table`.
+
+
+    .. seealso::
+
+        :ref:`schema_indexes` - General information on :class:`.Index`.
+
+        :ref:`postgresql_indexes` - PostgreSQL-specific options available for
+        the :class:`.Index` construct.
+
+        :ref:`mysql_indexes` - MySQL-specific options available for the
+        :class:`.Index` construct.
+
+        :ref:`mssql_indexes` - MSSQL-specific options available for the
+        :class:`.Index` construct.
+
+    """
+
+    __visit_name__ = "index"
+
+    def __init__(self, name, *expressions, **kw):
+        r"""Construct an index object.
+
+        :param name:
+          The name of the index
+
+        :param \*expressions:
+          Column expressions to include in the index.   The expressions
+          are normally instances of :class:`_schema.Column`, but may also
+          be arbitrary SQL expressions which ultimately refer to a
+          :class:`_schema.Column`.
+
+        :param unique=False:
+            Keyword only argument; if True, create a unique index.
+
+        :param quote=None:
+            Keyword only argument; whether to apply quoting to the name of
+            the index.  Works in the same manner as that of
+            :paramref:`_schema.Column.quote`.
+
+        :param info=None: Optional data dictionary which will be populated
+            into the :attr:`.SchemaItem.info` attribute of this object.
+
+            .. versionadded:: 1.0.0
+
+        :param \**kw: Additional keyword arguments not mentioned above are
+            dialect specific, and passed in the form
+            ``<dialectname>_<argname>``. See the documentation regarding an
+            individual dialect at :ref:`dialect_toplevel` for detail on
+            documented arguments.
+
+        """
+        self.table = table = None
+
+        self.name = quoted_name(name, kw.pop("quote", None))
+        self.unique = kw.pop("unique", False)
+        _column_flag = kw.pop("_column_flag", False)
+        if "info" in kw:
+            self.info = kw.pop("info")
+
+        # TODO: consider "table" argument being public, but for
+        # the purpose of the fix here, it starts as private.
+        if "_table" in kw:
+            table = kw.pop("_table")
+
+        self._validate_dialect_kwargs(kw)
+
+        self.expressions = []
+        # will call _set_parent() if table-bound column
+        # objects are present
+        ColumnCollectionMixin.__init__(
+            self,
+            *expressions,
+            _column_flag=_column_flag,
+            _gather_expressions=self.expressions
+        )
+
+        if table is not None:
+            self._set_parent(table)
+
+    def _set_parent(self, table, **kw):
+        ColumnCollectionMixin._set_parent(self, table)
+
+        if self.table is not None and table is not self.table:
+            raise exc.ArgumentError(
+                "Index '%s' is against table '%s', and "
+                "cannot be associated with table '%s'."
+                % (self.name, self.table.description, table.description)
+            )
+        self.table = table
+        table.indexes.add(self)
+
+        expressions = self.expressions
+        col_expressions = self._col_expressions(table)
+        assert len(expressions) == len(col_expressions)
+        self.expressions = [
+            expr if isinstance(expr, ClauseElement) else colexpr
+            for expr, colexpr in zip(expressions, col_expressions)
+        ]
+
+    @property
+    def bind(self):
+        """Return the connectable associated with this Index."""
+
+        return self.table.bind
+
+    def create(self, bind=None, checkfirst=False):
+        """Issue a ``CREATE`` statement for this
+        :class:`.Index`, using the given :class:`.Connectable`
+        for connectivity.
+
+        .. note:: the "bind" argument will be required in
+           SQLAlchemy 2.0.
+
+        .. seealso::
+
+            :meth:`_schema.MetaData.create_all`.
+
+        """
+        if bind is None:
+            bind = _bind_or_error(self)
+        bind._run_ddl_visitor(ddl.SchemaGenerator, self, checkfirst=checkfirst)
+        return self
+
+    def drop(self, bind=None, checkfirst=False):
+        """Issue a ``DROP`` statement for this
+        :class:`.Index`, using the given :class:`.Connectable`
+        for connectivity.
+
+        .. note:: the "bind" argument will be required in
+           SQLAlchemy 2.0.
+
+        .. seealso::
+
+            :meth:`_schema.MetaData.drop_all`.
+
+        """
+        if bind is None:
+            bind = _bind_or_error(self)
+        bind._run_ddl_visitor(ddl.SchemaDropper, self, checkfirst=checkfirst)
+
+    def __repr__(self):
+        return "Index(%s)" % (
+            ", ".join(
+                [repr(self.name)]
+                + [repr(e) for e in self.expressions]
+                + (self.unique and ["unique=True"] or [])
+            )
+        )
+
+
+DEFAULT_NAMING_CONVENTION = util.immutabledict({"ix": "ix_%(column_0_label)s"})
+
+
+class MetaData(SchemaItem):
+    """A collection of :class:`_schema.Table`
+    objects and their associated schema
+    constructs.
+
+    Holds a collection of :class:`_schema.Table` objects as well as
+    an optional binding to an :class:`_engine.Engine` or
+    :class:`_engine.Connection`.  If bound, the :class:`_schema.Table` objects
+    in the collection and their columns may participate in implicit SQL
+    execution.
+
+    The :class:`_schema.Table` objects themselves are stored in the
+    :attr:`_schema.MetaData.tables` dictionary.
+
+    :class:`_schema.MetaData` is a thread-safe object for read operations.
+    Construction of new tables within a single :class:`_schema.MetaData`
+    object,
+    either explicitly or via reflection, may not be completely thread-safe.
+
+    .. seealso::
+
+        :ref:`metadata_describing` - Introduction to database metadata
+
+    """
+
+    __visit_name__ = "metadata"
+
+    @util.deprecated_params(
+        bind=(
+            "2.0",
+            "The :paramref:`_schema.MetaData.bind` argument is deprecated and "
+            "will be removed in SQLAlchemy 2.0.",
+        ),
+    )
+    def __init__(
+        self,
+        bind=None,
+        schema=None,
+        quote_schema=None,
+        naming_convention=None,
+        info=None,
+    ):
+        """Create a new MetaData object.
+
+        :param bind:
+          An Engine or Connection to bind to.  May also be a string or URL
+          instance, these are passed to :func:`_sa.create_engine` and
+          this :class:`_schema.MetaData` will
+          be bound to the resulting engine.
+
+        :param schema:
+           The default schema to use for the :class:`_schema.Table`,
+           :class:`.Sequence`, and potentially other objects associated with
+           this :class:`_schema.MetaData`. Defaults to ``None``.
+
+           .. seealso::
+
+                :ref:`schema_metadata_schema_name` - details on how the
+                :paramref:`_schema.MetaData.schema` parameter is used.
+
+                :paramref:`_schema.Table.schema`
+
+                :paramref:`.Sequence.schema`
+
+        :param quote_schema:
+            Sets the ``quote_schema`` flag for those :class:`_schema.Table`,
+            :class:`.Sequence`, and other objects which make usage of the
+            local ``schema`` name.
+
+        :param info: Optional data dictionary which will be populated into the
+            :attr:`.SchemaItem.info` attribute of this object.
+
+            .. versionadded:: 1.0.0
+
+        :param naming_convention: a dictionary referring to values which
+          will establish default naming conventions for :class:`.Constraint`
+          and :class:`.Index` objects, for those objects which are not given
+          a name explicitly.
+
+          The keys of this dictionary may be:
+
+          * a constraint or Index class, e.g. the :class:`.UniqueConstraint`,
+            :class:`_schema.ForeignKeyConstraint` class, the :class:`.Index`
+            class
+
+          * a string mnemonic for one of the known constraint classes;
+            ``"fk"``, ``"pk"``, ``"ix"``, ``"ck"``, ``"uq"`` for foreign key,
+            primary key, index, check, and unique constraint, respectively.
+
+          * the string name of a user-defined "token" that can be used
+            to define new naming tokens.
+
+          The values associated with each "constraint class" or "constraint
+          mnemonic" key are string naming templates, such as
+          ``"uq_%(table_name)s_%(column_0_name)s"``,
+          which describe how the name should be composed.  The values
+          associated with user-defined "token" keys should be callables of the
+          form ``fn(constraint, table)``, which accepts the constraint/index
+          object and :class:`_schema.Table` as arguments, returning a string
+          result.
+
+          The built-in names are as follows, some of which may only be
+          available for certain types of constraint:
+
+            * ``%(table_name)s`` - the name of the :class:`_schema.Table`
+              object
+              associated with the constraint.
+
+            * ``%(referred_table_name)s`` - the name of the
+              :class:`_schema.Table`
+              object associated with the referencing target of a
+              :class:`_schema.ForeignKeyConstraint`.
+
+            * ``%(column_0_name)s`` - the name of the :class:`_schema.Column`
+              at
+              index position "0" within the constraint.
+
+            * ``%(column_0N_name)s`` - the name of all :class:`_schema.Column`
+              objects in order within the constraint, joined without a
+              separator.
+
+            * ``%(column_0_N_name)s`` - the name of all
+              :class:`_schema.Column`
+              objects in order within the constraint, joined with an
+              underscore as a separator.
+
+            * ``%(column_0_label)s``, ``%(column_0N_label)s``,
+              ``%(column_0_N_label)s`` - the label of either the zeroth
+              :class:`_schema.Column` or all :class:`.Columns`, separated with
+              or without an underscore
+
+            * ``%(column_0_key)s``, ``%(column_0N_key)s``,
+              ``%(column_0_N_key)s`` - the key of either the zeroth
+              :class:`_schema.Column` or all :class:`.Columns`, separated with
+              or without an underscore
+
+            * ``%(referred_column_0_name)s``, ``%(referred_column_0N_name)s``
+              ``%(referred_column_0_N_name)s``,  ``%(referred_column_0_key)s``,
+              ``%(referred_column_0N_key)s``, ...  column tokens which
+              render the names/keys/labels of columns that are referenced
+              by a  :class:`_schema.ForeignKeyConstraint`.
+
+            * ``%(constraint_name)s`` - a special key that refers to the
+              existing name given to the constraint.  When this key is
+              present, the :class:`.Constraint` object's existing name will be
+              replaced with one that is composed from template string that
+              uses this token. When this token is present, it is required that
+              the :class:`.Constraint` is given an explicit name ahead of time.
+
+            * user-defined: any additional token may be implemented by passing
+              it along with a ``fn(constraint, table)`` callable to the
+              naming_convention dictionary.
+
+          .. versionadded:: 1.3.0 - added new ``%(column_0N_name)s``,
+             ``%(column_0_N_name)s``, and related tokens that produce
+             concatenations of names, keys, or labels for all columns referred
+             to by a given constraint.
+
+          .. seealso::
+
+                :ref:`constraint_naming_conventions` - for detailed usage
+                examples.
+
+        """
+        self.tables = util.FacadeDict()
+        self.schema = quoted_name(schema, quote_schema)
+        self.naming_convention = (
+            naming_convention
+            if naming_convention
+            else DEFAULT_NAMING_CONVENTION
+        )
+        if info:
+            self.info = info
+        self._schemas = set()
+        self._sequences = {}
+        self._fk_memos = collections.defaultdict(list)
+
+        self.bind = bind
+
+    tables = None
+    """A dictionary of :class:`_schema.Table`
+    objects keyed to their name or "table key".
+
+    The exact key is that determined by the :attr:`_schema.Table.key`
+    attribute;
+    for a table with no :attr:`_schema.Table.schema` attribute,
+    this is the same
+    as :attr:`_schema.Table.name`.  For a table with a schema,
+    it is typically of the
+    form ``schemaname.tablename``.
+
+    .. seealso::
+
+        :attr:`_schema.MetaData.sorted_tables`
+
+    """
+
+    def __repr__(self):
+        if self.bind:
+            return "MetaData(bind=%r)" % self.bind
+        else:
+            return "MetaData()"
+
+    def __contains__(self, table_or_key):
+        if not isinstance(table_or_key, util.string_types):
+            table_or_key = table_or_key.key
+        return table_or_key in self.tables
+
+    def _add_table(self, name, schema, table):
+        key = _get_table_key(name, schema)
+        self.tables._insert_item(key, table)
+        if schema:
+            self._schemas.add(schema)
+
+    def _remove_table(self, name, schema):
+        key = _get_table_key(name, schema)
+        removed = dict.pop(self.tables, key, None)
+        if removed is not None:
+            for fk in removed.foreign_keys:
+                fk._remove_from_metadata(self)
+        if self._schemas:
+            self._schemas = set(
+                [
+                    t.schema
+                    for t in self.tables.values()
+                    if t.schema is not None
+                ]
+            )
+
+    def __getstate__(self):
+        return {
+            "tables": self.tables,
+            "schema": self.schema,
+            "schemas": self._schemas,
+            "sequences": self._sequences,
+            "fk_memos": self._fk_memos,
+            "naming_convention": self.naming_convention,
+        }
+
+    def __setstate__(self, state):
+        self.tables = state["tables"]
+        self.schema = state["schema"]
+        self.naming_convention = state["naming_convention"]
+        self._bind = None
+        self._sequences = state["sequences"]
+        self._schemas = state["schemas"]
+        self._fk_memos = state["fk_memos"]
+
+    def is_bound(self):
+        """True if this MetaData is bound to an Engine or Connection."""
+
+        return self._bind is not None
+
+    def bind(self):
+        """An :class:`_engine.Engine` or :class:`_engine.Connection`
+        to which this
+        :class:`_schema.MetaData` is bound.
+
+        Typically, a :class:`_engine.Engine` is assigned to this attribute
+        so that "implicit execution" may be used, or alternatively
+        as a means of providing engine binding information to an
+        ORM :class:`.Session` object::
+
+            engine = create_engine("someurl://")
+            metadata.bind = engine
+
+        .. deprecated :: 1.4
+
+            The metadata.bind attribute, as part of the deprecated system
+            of "implicit execution", is itself deprecated and will be
+            removed in SQLAlchemy 2.0.
+
+        .. seealso::
+
+           :ref:`dbengine_implicit` - background on "bound metadata"
+
+        """
+        return self._bind
+
+    @util.preload_module("sqlalchemy.engine.url")
+    def _bind_to(self, bind):
+        """Bind this MetaData to an Engine, Connection, string or URL."""
+        url = util.preloaded.engine_url
+        if isinstance(bind, util.string_types + (url.URL,)):
+            self._bind = sqlalchemy.create_engine(bind)
+        else:
+            self._bind = bind
+
+    bind = property(bind, _bind_to)
+
+    def clear(self):
+        """Clear all Table objects from this MetaData."""
+
+        dict.clear(self.tables)
+        self._schemas.clear()
+        self._fk_memos.clear()
+
+    def remove(self, table):
+        """Remove the given Table object from this MetaData."""
+
+        self._remove_table(table.name, table.schema)
+
+    @property
+    def sorted_tables(self):
+        """Returns a list of :class:`_schema.Table` objects sorted in order of
+        foreign key dependency.
+
+        The sorting will place :class:`_schema.Table`
+        objects that have dependencies
+        first, before the dependencies themselves, representing the
+        order in which they can be created.   To get the order in which
+        the tables would be dropped, use the ``reversed()`` Python built-in.
+
+        .. warning::
+
+            The :attr:`.MetaData.sorted_tables` attribute cannot by itself
+            accommodate automatic resolution of dependency cycles between
+            tables, which are usually caused by mutually dependent foreign key
+            constraints. When these cycles are detected, the foreign keys
+            of these tables are omitted from consideration in the sort.
+            A warning is emitted when this condition occurs, which will be an
+            exception raise in a future release.   Tables which are not part
+            of the cycle will still be returned in dependency order.
+
+            To resolve these cycles, the
+            :paramref:`_schema.ForeignKeyConstraint.use_alter` parameter may be
+            applied to those constraints which create a cycle.  Alternatively,
+            the :func:`_schema.sort_tables_and_constraints` function will
+            automatically return foreign key constraints in a separate
+            collection when cycles are detected so that they may be applied
+            to a schema separately.
+
+            .. versionchanged:: 1.3.17 - a warning is emitted when
+               :attr:`.MetaData.sorted_tables` cannot perform a proper sort
+               due to cyclical dependencies.  This will be an exception in a
+               future release.  Additionally, the sort will continue to return
+               other tables not involved in the cycle in dependency order which
+               was not the case previously.
+
+        .. seealso::
+
+            :func:`_schema.sort_tables`
+
+            :func:`_schema.sort_tables_and_constraints`
+
+            :attr:`_schema.MetaData.tables`
+
+            :meth:`_reflection.Inspector.get_table_names`
+
+            :meth:`_reflection.Inspector.get_sorted_table_and_fkc_names`
+
+
+        """
+        return ddl.sort_tables(
+            sorted(self.tables.values(), key=lambda t: t.key)
+        )
+
+    def reflect(
+        self,
+        bind=None,
+        schema=None,
+        views=False,
+        only=None,
+        extend_existing=False,
+        autoload_replace=True,
+        resolve_fks=True,
+        **dialect_kwargs
+    ):
+        r"""Load all available table definitions from the database.
+
+        Automatically creates ``Table`` entries in this ``MetaData`` for any
+        table available in the database but not yet present in the
+        ``MetaData``.  May be called multiple times to pick up tables recently
+        added to the database, however no special action is taken if a table
+        in this ``MetaData`` no longer exists in the database.
+
+        :param bind:
+          A :class:`.Connectable` used to access the database; if None, uses
+          the existing bind on this ``MetaData``, if any.
+
+          .. note:: the "bind" argument will be required in
+             SQLAlchemy 2.0.
+
+        :param schema:
+          Optional, query and reflect tables from an alternate schema.
+          If None, the schema associated with this :class:`_schema.MetaData`
+          is used, if any.
+
+        :param views:
+          If True, also reflect views.
+
+        :param only:
+          Optional.  Load only a sub-set of available named tables.  May be
+          specified as a sequence of names or a callable.
+
+          If a sequence of names is provided, only those tables will be
+          reflected.  An error is raised if a table is requested but not
+          available.  Named tables already present in this ``MetaData`` are
+          ignored.
+
+          If a callable is provided, it will be used as a boolean predicate to
+          filter the list of potential table names.  The callable is called
+          with a table name and this ``MetaData`` instance as positional
+          arguments and should return a true value for any table to reflect.
+
+        :param extend_existing: Passed along to each :class:`_schema.Table` as
+          :paramref:`_schema.Table.extend_existing`.
+
+          .. versionadded:: 0.9.1
+
+        :param autoload_replace: Passed along to each :class:`_schema.Table`
+          as
+          :paramref:`_schema.Table.autoload_replace`.
+
+          .. versionadded:: 0.9.1
+
+        :param resolve_fks: if True, reflect :class:`_schema.Table`
+         objects linked
+         to :class:`_schema.ForeignKey` objects located in each
+         :class:`_schema.Table`.
+         For :meth:`_schema.MetaData.reflect`,
+         this has the effect of reflecting
+         related tables that might otherwise not be in the list of tables
+         being reflected, for example if the referenced table is in a
+         different schema or is omitted via the
+         :paramref:`.MetaData.reflect.only` parameter.  When False,
+         :class:`_schema.ForeignKey` objects are not followed to the
+         :class:`_schema.Table`
+         in which they link, however if the related table is also part of the
+         list of tables that would be reflected in any case, the
+         :class:`_schema.ForeignKey` object will still resolve to its related
+         :class:`_schema.Table` after the :meth:`_schema.MetaData.reflect`
+         operation is
+         complete.   Defaults to True.
+
+         .. versionadded:: 1.3.0
+
+         .. seealso::
+
+            :paramref:`_schema.Table.resolve_fks`
+
+        :param \**dialect_kwargs: Additional keyword arguments not mentioned
+         above are dialect specific, and passed in the form
+         ``<dialectname>_<argname>``.  See the documentation regarding an
+         individual dialect at :ref:`dialect_toplevel` for detail on
+         documented arguments.
+
+          .. versionadded:: 0.9.2 - Added
+             :paramref:`.MetaData.reflect.**dialect_kwargs` to support
+             dialect-level reflection options for all :class:`_schema.Table`
+             objects reflected.
+
+        """
+        if bind is None:
+            bind = _bind_or_error(self)
+
+        with inspection.inspect(bind)._inspection_context() as insp:
+            reflect_opts = {
+                "autoload_with": insp,
+                "extend_existing": extend_existing,
+                "autoload_replace": autoload_replace,
+                "resolve_fks": resolve_fks,
+                "_extend_on": set(),
+            }
+
+            reflect_opts.update(dialect_kwargs)
+
+            if schema is None:
+                schema = self.schema
+
+            if schema is not None:
+                reflect_opts["schema"] = schema
+
+            available = util.OrderedSet(insp.get_table_names(schema))
+            if views:
+                available.update(insp.get_view_names(schema))
+
+            if schema is not None:
+                available_w_schema = util.OrderedSet(
+                    ["%s.%s" % (schema, name) for name in available]
+                )
+            else:
+                available_w_schema = available
+
+            current = set(self.tables)
+
+            if only is None:
+                load = [
+                    name
+                    for name, schname in zip(available, available_w_schema)
+                    if extend_existing or schname not in current
+                ]
+            elif callable(only):
+                load = [
+                    name
+                    for name, schname in zip(available, available_w_schema)
+                    if (extend_existing or schname not in current)
+                    and only(name, self)
+                ]
+            else:
+                missing = [name for name in only if name not in available]
+                if missing:
+                    s = schema and (" schema '%s'" % schema) or ""
+                    raise exc.InvalidRequestError(
+                        "Could not reflect: requested table(s) not available "
+                        "in %r%s: (%s)" % (bind.engine, s, ", ".join(missing))
+                    )
+                load = [
+                    name
+                    for name in only
+                    if extend_existing or name not in current
+                ]
+
+            for name in load:
+                try:
+                    Table(name, self, **reflect_opts)
+                except exc.UnreflectableTableError as uerr:
+                    util.warn("Skipping table %s: %s" % (name, uerr))
+
+    def create_all(self, bind=None, tables=None, checkfirst=True):
+        """Create all tables stored in this metadata.
+
+        Conditional by default, will not attempt to recreate tables already
+        present in the target database.
+
+        :param bind:
+          A :class:`.Connectable` used to access the
+          database; if None, uses the existing bind on this ``MetaData``, if
+          any.
+
+          .. note:: the "bind" argument will be required in
+             SQLAlchemy 2.0.
+
+        :param tables:
+          Optional list of ``Table`` objects, which is a subset of the total
+          tables in the ``MetaData`` (others are ignored).
+
+        :param checkfirst:
+          Defaults to True, don't issue CREATEs for tables already present
+          in the target database.
+
+        """
+        if bind is None:
+            bind = _bind_or_error(self)
+        bind._run_ddl_visitor(
+            ddl.SchemaGenerator, self, checkfirst=checkfirst, tables=tables
+        )
+
+    def drop_all(self, bind=None, tables=None, checkfirst=True):
+        """Drop all tables stored in this metadata.
+
+        Conditional by default, will not attempt to drop tables not present in
+        the target database.
+
+        :param bind:
+          A :class:`.Connectable` used to access the
+          database; if None, uses the existing bind on this ``MetaData``, if
+          any.
+
+          .. note:: the "bind" argument will be required in
+             SQLAlchemy 2.0.
+
+        :param tables:
+          Optional list of ``Table`` objects, which is a subset of the
+          total tables in the ``MetaData`` (others are ignored).
+
+        :param checkfirst:
+          Defaults to True, only issue DROPs for tables confirmed to be
+          present in the target database.
+
+        """
+        if bind is None:
+            bind = _bind_or_error(self)
+        bind._run_ddl_visitor(
+            ddl.SchemaDropper, self, checkfirst=checkfirst, tables=tables
+        )
+
+
+@util.deprecated_cls(
+    "1.4",
+    ":class:`.ThreadLocalMetaData` is deprecated and will be removed "
+    "in a future release.",
+    constructor="__init__",
+)
+class ThreadLocalMetaData(MetaData):
+    """A MetaData variant that presents a different ``bind`` in every thread.
+
+    Makes the ``bind`` property of the MetaData a thread-local value, allowing
+    this collection of tables to be bound to different ``Engine``
+    implementations or connections in each thread.
+
+    The ThreadLocalMetaData starts off bound to None in each thread.  Binds
+    must be made explicitly by assigning to the ``bind`` property or using
+    ``connect()``.  You can also re-bind dynamically multiple times per
+    thread, just like a regular ``MetaData``.
+
+    """
+
+    __visit_name__ = "metadata"
+
+    def __init__(self):
+        """Construct a ThreadLocalMetaData."""
+
+        self.context = util.threading.local()
+        self.__engines = {}
+        super(ThreadLocalMetaData, self).__init__()
+
+    def bind(self):
+        """The bound Engine or Connection for this thread.
+
+        This property may be assigned an Engine or Connection, or assigned a
+        string or URL to automatically create a basic Engine for this bind
+        with ``create_engine()``."""
+
+        return getattr(self.context, "_engine", None)
+
+    @util.preload_module("sqlalchemy.engine.url")
+    def _bind_to(self, bind):
+        """Bind to a Connectable in the caller's thread."""
+        url = util.preloaded.engine_url
+        if isinstance(bind, util.string_types + (url.URL,)):
+            try:
+                self.context._engine = self.__engines[bind]
+            except KeyError:
+                e = sqlalchemy.create_engine(bind)
+                self.__engines[bind] = e
+                self.context._engine = e
+        else:
+            # TODO: this is squirrely.  we shouldn't have to hold onto engines
+            # in a case like this
+            if bind not in self.__engines:
+                self.__engines[bind] = bind
+            self.context._engine = bind
+
+    bind = property(bind, _bind_to)
+
+    def is_bound(self):
+        """True if there is a bind for this thread."""
+        return (
+            hasattr(self.context, "_engine")
+            and self.context._engine is not None
+        )
+
+    def dispose(self):
+        """Dispose all bound engines, in all thread contexts."""
+
+        for e in self.__engines.values():
+            if hasattr(e, "dispose"):
+                e.dispose()
+
+
+class Computed(FetchedValue, SchemaItem):
+    """Defines a generated column, i.e. "GENERATED ALWAYS AS" syntax.
+
+    The :class:`.Computed` construct is an inline construct added to the
+    argument list of a :class:`_schema.Column` object::
+
+        from sqlalchemy import Computed
+
+        Table('square', metadata_obj,
+            Column('side', Float, nullable=False),
+            Column('area', Float, Computed('side * side'))
+        )
+
+    See the linked documentation below for complete details.
+
+    .. versionadded:: 1.3.11
+
+    .. seealso::
+
+        :ref:`computed_ddl`
+
+    """
+
+    __visit_name__ = "computed_column"
+
+    @_document_text_coercion(
+        "sqltext", ":class:`.Computed`", ":paramref:`.Computed.sqltext`"
+    )
+    def __init__(self, sqltext, persisted=None):
+        """Construct a GENERATED ALWAYS AS DDL construct to accompany a
+        :class:`_schema.Column`.
+
+        :param sqltext:
+          A string containing the column generation expression, which will be
+          used verbatim, or a SQL expression construct, such as a
+          :func:`_expression.text`
+          object.   If given as a string, the object is converted to a
+          :func:`_expression.text` object.
+
+        :param persisted:
+          Optional, controls how this column should be persisted by the
+          database.   Possible values are:
+
+          * ``None``, the default, it will use the default persistence
+            defined by the database.
+          * ``True``, will render ``GENERATED ALWAYS AS ... STORED``, or the
+            equivalent for the target database if supported.
+          * ``False``, will render ``GENERATED ALWAYS AS ... VIRTUAL``, or
+            the equivalent for the target database if supported.
+
+          Specifying ``True`` or ``False`` may raise an error when the DDL
+          is emitted to the target database if the database does not support
+          that persistence option.   Leaving this parameter at its default
+          of ``None`` is guaranteed to succeed for all databases that support
+          ``GENERATED ALWAYS AS``.
+
+        """
+        self.sqltext = coercions.expect(roles.DDLExpressionRole, sqltext)
+        self.persisted = persisted
+        self.column = None
+
+    def _set_parent(self, parent, **kw):
+        if not isinstance(
+            parent.server_default, (type(None), Computed)
+        ) or not isinstance(parent.server_onupdate, (type(None), Computed)):
+            raise exc.ArgumentError(
+                "A generated column cannot specify a server_default or a "
+                "server_onupdate argument"
+            )
+        self.column = parent
+        parent.computed = self
+        self.column.server_onupdate = self
+        self.column.server_default = self
+
+    def _as_for_update(self, for_update):
+        return self
+
+    @util.deprecated(
+        "1.4",
+        "The :meth:`_schema.Computed.copy` method is deprecated "
+        "and will be removed in a future release.",
+    )
+    def copy(self, target_table=None, **kw):
+        return self._copy(target_table, **kw)
+
+    def _copy(self, target_table=None, **kw):
+        sqltext = _copy_expression(
+            self.sqltext,
+            self.column.table if self.column is not None else None,
+            target_table,
+        )
+        g = Computed(sqltext, persisted=self.persisted)
+
+        return self._schema_item_copy(g)
+
+
+class Identity(IdentityOptions, FetchedValue, SchemaItem):
+    """Defines an identity column, i.e. "GENERATED { ALWAYS | BY DEFAULT }
+    AS IDENTITY" syntax.
+
+    The :class:`.Identity` construct is an inline construct added to the
+    argument list of a :class:`_schema.Column` object::
+
+        from sqlalchemy import Identity
+
+        Table('foo', metadata_obj,
+            Column('id', Integer, Identity())
+            Column('description', Text),
+        )
+
+    See the linked documentation below for complete details.
+
+    .. versionadded:: 1.4
+
+    .. seealso::
+
+        :ref:`identity_ddl`
+
+    """
+
+    __visit_name__ = "identity_column"
+
+    def __init__(
+        self,
+        always=False,
+        on_null=None,
+        start=None,
+        increment=None,
+        minvalue=None,
+        maxvalue=None,
+        nominvalue=None,
+        nomaxvalue=None,
+        cycle=None,
+        cache=None,
+        order=None,
+    ):
+        """Construct a GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY DDL
+        construct to accompany a :class:`_schema.Column`.
+
+        See the :class:`.Sequence` documentation for a complete description
+        of most parameters.
+
+        .. note::
+            MSSQL supports this construct as the preferred alternative to
+            generate an IDENTITY on a column, but it uses non standard
+            syntax that only support :paramref:`_schema.Identity.start`
+            and :paramref:`_schema.Identity.increment`.
+            All other parameters are ignored.
+
+        :param always:
+          A boolean, that indicates the type of identity column.
+          If ``False`` is specified, the default, then the user-specified
+          value takes precedence.
+          If ``True`` is specified, a user-specified value is not accepted (
+          on some backends, like PostgreSQL, OVERRIDING SYSTEM VALUE, or
+          similar, may be specified in an INSERT to override the sequence
+          value).
+          Some backends also have a default value for this parameter,
+          ``None`` can be used to omit rendering this part in the DDL. It
+          will be treated as ``False`` if a backend does not have a default
+          value.
+
+        :param on_null:
+          Set to ``True`` to specify ON NULL in conjunction with a
+          ``always=False`` identity column. This option is only supported on
+          some backends, like Oracle.
+
+        :param start: the starting index of the sequence.
+        :param increment: the increment value of the sequence.
+        :param minvalue: the minimum value of the sequence.
+        :param maxvalue: the maximum value of the sequence.
+        :param nominvalue: no minimum value of the sequence.
+        :param nomaxvalue: no maximum value of the sequence.
+        :param cycle: allows the sequence to wrap around when the maxvalue
+         or minvalue has been reached.
+        :param cache: optional integer value; number of future values in the
+         sequence which are calculated in advance.
+        :param order: optional boolean value; if true, renders the
+         ORDER keyword.
+
+        """
+        IdentityOptions.__init__(
+            self,
+            start=start,
+            increment=increment,
+            minvalue=minvalue,
+            maxvalue=maxvalue,
+            nominvalue=nominvalue,
+            nomaxvalue=nomaxvalue,
+            cycle=cycle,
+            cache=cache,
+            order=order,
+        )
+        self.always = always
+        self.on_null = on_null
+        self.column = None
+
+    def _set_parent(self, parent, **kw):
+        if not isinstance(
+            parent.server_default, (type(None), Identity)
+        ) or not isinstance(parent.server_onupdate, type(None)):
+            raise exc.ArgumentError(
+                "A column with an Identity object cannot specify a "
+                "server_default or a server_onupdate argument"
+            )
+        if parent.autoincrement is False:
+            raise exc.ArgumentError(
+                "A column with an Identity object cannot specify "
+                "autoincrement=False"
+            )
+        self.column = parent
+
+        parent.identity = self
+        if parent._user_defined_nullable is NULL_UNSPECIFIED:
+            parent.nullable = False
+
+        parent.server_default = self
+
+    def _as_for_update(self, for_update):
+        return self
+
+    @util.deprecated(
+        "1.4",
+        "The :meth:`_schema.Identity.copy` method is deprecated "
+        "and will be removed in a future release.",
+    )
+    def copy(self, **kw):
+        return self._copy(**kw)
+
+    def _copy(self, **kw):
+        i = Identity(
+            always=self.always,
+            on_null=self.on_null,
+            start=self.start,
+            increment=self.increment,
+            minvalue=self.minvalue,
+            maxvalue=self.maxvalue,
+            nominvalue=self.nominvalue,
+            nomaxvalue=self.nomaxvalue,
+            cycle=self.cycle,
+            cache=self.cache,
+            order=self.order,
+        )
+
+        return self._schema_item_copy(i)