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diff --git a/lib/sqlalchemy/ext/associationproxy.py b/lib/sqlalchemy/ext/associationproxy.py
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+# ext/associationproxy.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
+
+"""Contain the ``AssociationProxy`` class.
+
+The ``AssociationProxy`` is a Python property object which provides
+transparent proxied access to the endpoint of an association object.
+
+See the example ``examples/association/proxied_association.py``.
+
+"""
+import operator
+
+from .. import exc
+from .. import inspect
+from .. import orm
+from .. import util
+from ..orm import collections
+from ..orm import interfaces
+from ..sql import or_
+from ..sql.operators import ColumnOperators
+
+
+def association_proxy(target_collection, attr, **kw):
+    r"""Return a Python property implementing a view of a target
+    attribute which references an attribute on members of the
+    target.
+
+    The returned value is an instance of :class:`.AssociationProxy`.
+
+    Implements a Python property representing a relationship as a collection
+    of simpler values, or a scalar value.  The proxied property will mimic
+    the collection type of the target (list, dict or set), or, in the case of
+    a one to one relationship, a simple scalar value.
+
+    :param target_collection: Name of the attribute we'll proxy to.
+      This attribute is typically mapped by
+      :func:`~sqlalchemy.orm.relationship` to link to a target collection, but
+      can also be a many-to-one or non-scalar relationship.
+
+    :param attr: Attribute on the associated instance or instances we'll
+      proxy for.
+
+      For example, given a target collection of [obj1, obj2], a list created
+      by this proxy property would look like [getattr(obj1, *attr*),
+      getattr(obj2, *attr*)]
+
+      If the relationship is one-to-one or otherwise uselist=False, then
+      simply: getattr(obj, *attr*)
+
+    :param creator: optional.
+
+      When new items are added to this proxied collection, new instances of
+      the class collected by the target collection will be created.  For list
+      and set collections, the target class constructor will be called with
+      the 'value' for the new instance.  For dict types, two arguments are
+      passed: key and value.
+
+      If you want to construct instances differently, supply a *creator*
+      function that takes arguments as above and returns instances.
+
+      For scalar relationships, creator() will be called if the target is None.
+      If the target is present, set operations are proxied to setattr() on the
+      associated object.
+
+      If you have an associated object with multiple attributes, you may set
+      up multiple association proxies mapping to different attributes.  See
+      the unit tests for examples, and for examples of how creator() functions
+      can be used to construct the scalar relationship on-demand in this
+      situation.
+
+    :param \*\*kw: Passes along any other keyword arguments to
+      :class:`.AssociationProxy`.
+
+    """
+    return AssociationProxy(target_collection, attr, **kw)
+
+
+ASSOCIATION_PROXY = util.symbol("ASSOCIATION_PROXY")
+"""Symbol indicating an :class:`.InspectionAttr` that's
+    of type :class:`.AssociationProxy`.
+
+   Is assigned to the :attr:`.InspectionAttr.extension_type`
+   attribute.
+
+"""
+
+
+class AssociationProxy(interfaces.InspectionAttrInfo):
+    """A descriptor that presents a read/write view of an object attribute."""
+
+    is_attribute = True
+    extension_type = ASSOCIATION_PROXY
+
+    def __init__(
+        self,
+        target_collection,
+        attr,
+        creator=None,
+        getset_factory=None,
+        proxy_factory=None,
+        proxy_bulk_set=None,
+        info=None,
+        cascade_scalar_deletes=False,
+    ):
+        """Construct a new :class:`.AssociationProxy`.
+
+        The :func:`.association_proxy` function is provided as the usual
+        entrypoint here, though :class:`.AssociationProxy` can be instantiated
+        and/or subclassed directly.
+
+        :param target_collection: Name of the collection we'll proxy to,
+          usually created with :func:`_orm.relationship`.
+
+        :param attr: Attribute on the collected instances we'll proxy
+          for.  For example, given a target collection of [obj1, obj2], a
+          list created by this proxy property would look like
+          [getattr(obj1, attr), getattr(obj2, attr)]
+
+        :param creator: Optional. When new items are added to this proxied
+          collection, new instances of the class collected by the target
+          collection will be created.  For list and set collections, the
+          target class constructor will be called with the 'value' for the
+          new instance.  For dict types, two arguments are passed:
+          key and value.
+
+          If you want to construct instances differently, supply a 'creator'
+          function that takes arguments as above and returns instances.
+
+        :param cascade_scalar_deletes: when True, indicates that setting
+         the proxied value to ``None``, or deleting it via ``del``, should
+         also remove the source object.  Only applies to scalar attributes.
+         Normally, removing the proxied target will not remove the proxy
+         source, as this object may have other state that is still to be
+         kept.
+
+         .. versionadded:: 1.3
+
+         .. seealso::
+
+            :ref:`cascade_scalar_deletes` - complete usage example
+
+        :param getset_factory: Optional.  Proxied attribute access is
+          automatically handled by routines that get and set values based on
+          the `attr` argument for this proxy.
+
+          If you would like to customize this behavior, you may supply a
+          `getset_factory` callable that produces a tuple of `getter` and
+          `setter` functions.  The factory is called with two arguments, the
+          abstract type of the underlying collection and this proxy instance.
+
+        :param proxy_factory: Optional.  The type of collection to emulate is
+          determined by sniffing the target collection.  If your collection
+          type can't be determined by duck typing or you'd like to use a
+          different collection implementation, you may supply a factory
+          function to produce those collections.  Only applicable to
+          non-scalar relationships.
+
+        :param proxy_bulk_set: Optional, use with proxy_factory.  See
+          the _set() method for details.
+
+        :param info: optional, will be assigned to
+         :attr:`.AssociationProxy.info` if present.
+
+         .. versionadded:: 1.0.9
+
+        """
+        self.target_collection = target_collection
+        self.value_attr = attr
+        self.creator = creator
+        self.getset_factory = getset_factory
+        self.proxy_factory = proxy_factory
+        self.proxy_bulk_set = proxy_bulk_set
+        self.cascade_scalar_deletes = cascade_scalar_deletes
+
+        self.key = "_%s_%s_%s" % (
+            type(self).__name__,
+            target_collection,
+            id(self),
+        )
+        if info:
+            self.info = info
+
+    def __get__(self, obj, class_):
+        if class_ is None:
+            return self
+        inst = self._as_instance(class_, obj)
+        if inst:
+            return inst.get(obj)
+
+        # obj has to be None here
+        # assert obj is None
+
+        return self
+
+    def __set__(self, obj, values):
+        class_ = type(obj)
+        return self._as_instance(class_, obj).set(obj, values)
+
+    def __delete__(self, obj):
+        class_ = type(obj)
+        return self._as_instance(class_, obj).delete(obj)
+
+    def for_class(self, class_, obj=None):
+        r"""Return the internal state local to a specific mapped class.
+
+        E.g., given a class ``User``::
+
+            class User(Base):
+                # ...
+
+                keywords = association_proxy('kws', 'keyword')
+
+        If we access this :class:`.AssociationProxy` from
+        :attr:`_orm.Mapper.all_orm_descriptors`, and we want to view the
+        target class for this proxy as mapped by ``User``::
+
+            inspect(User).all_orm_descriptors["keywords"].for_class(User).target_class
+
+        This returns an instance of :class:`.AssociationProxyInstance` that
+        is specific to the ``User`` class.   The :class:`.AssociationProxy`
+        object remains agnostic of its parent class.
+
+        :param class\_: the class that we are returning state for.
+
+        :param obj: optional, an instance of the class that is required
+         if the attribute refers to a polymorphic target, e.g. where we have
+         to look at the type of the actual destination object to get the
+         complete path.
+
+        .. versionadded:: 1.3 - :class:`.AssociationProxy` no longer stores
+           any state specific to a particular parent class; the state is now
+           stored in per-class :class:`.AssociationProxyInstance` objects.
+
+
+        """
+        return self._as_instance(class_, obj)
+
+    def _as_instance(self, class_, obj):
+        try:
+            inst = class_.__dict__[self.key + "_inst"]
+        except KeyError:
+            inst = None
+
+        # avoid exception context
+        if inst is None:
+            owner = self._calc_owner(class_)
+            if owner is not None:
+                inst = AssociationProxyInstance.for_proxy(self, owner, obj)
+                setattr(class_, self.key + "_inst", inst)
+            else:
+                inst = None
+
+        if inst is not None and not inst._is_canonical:
+            # the AssociationProxyInstance can't be generalized
+            # since the proxied attribute is not on the targeted
+            # class, only on subclasses of it, which might be
+            # different.  only return for the specific
+            # object's current value
+            return inst._non_canonical_get_for_object(obj)
+        else:
+            return inst
+
+    def _calc_owner(self, target_cls):
+        # we might be getting invoked for a subclass
+        # that is not mapped yet, in some declarative situations.
+        # save until we are mapped
+        try:
+            insp = inspect(target_cls)
+        except exc.NoInspectionAvailable:
+            # can't find a mapper, don't set owner. if we are a not-yet-mapped
+            # subclass, we can also scan through __mro__ to find a mapped
+            # class, but instead just wait for us to be called again against a
+            # mapped class normally.
+            return None
+        else:
+            return insp.mapper.class_manager.class_
+
+    def _default_getset(self, collection_class):
+        attr = self.value_attr
+        _getter = operator.attrgetter(attr)
+
+        def getter(target):
+            return _getter(target) if target is not None else None
+
+        if collection_class is dict:
+
+            def setter(o, k, v):
+                setattr(o, attr, v)
+
+        else:
+
+            def setter(o, v):
+                setattr(o, attr, v)
+
+        return getter, setter
+
+    def __repr__(self):
+        return "AssociationProxy(%r, %r)" % (
+            self.target_collection,
+            self.value_attr,
+        )
+
+
+class AssociationProxyInstance(object):
+    """A per-class object that serves class- and object-specific results.
+
+    This is used by :class:`.AssociationProxy` when it is invoked
+    in terms of a specific class or instance of a class, i.e. when it is
+    used as a regular Python descriptor.
+
+    When referring to the :class:`.AssociationProxy` as a normal Python
+    descriptor, the :class:`.AssociationProxyInstance` is the object that
+    actually serves the information.   Under normal circumstances, its presence
+    is transparent::
+
+        >>> User.keywords.scalar
+        False
+
+    In the special case that the :class:`.AssociationProxy` object is being
+    accessed directly, in order to get an explicit handle to the
+    :class:`.AssociationProxyInstance`, use the
+    :meth:`.AssociationProxy.for_class` method::
+
+        proxy_state = inspect(User).all_orm_descriptors["keywords"].for_class(User)
+
+        # view if proxy object is scalar or not
+        >>> proxy_state.scalar
+        False
+
+    .. versionadded:: 1.3
+
+    """  # noqa
+
+    def __init__(self, parent, owning_class, target_class, value_attr):
+        self.parent = parent
+        self.key = parent.key
+        self.owning_class = owning_class
+        self.target_collection = parent.target_collection
+        self.collection_class = None
+        self.target_class = target_class
+        self.value_attr = value_attr
+
+    target_class = None
+    """The intermediary class handled by this
+    :class:`.AssociationProxyInstance`.
+
+    Intercepted append/set/assignment events will result
+    in the generation of new instances of this class.
+
+    """
+
+    @classmethod
+    def for_proxy(cls, parent, owning_class, parent_instance):
+        target_collection = parent.target_collection
+        value_attr = parent.value_attr
+        prop = orm.class_mapper(owning_class).get_property(target_collection)
+
+        # this was never asserted before but this should be made clear.
+        if not isinstance(prop, orm.RelationshipProperty):
+            util.raise_(
+                NotImplementedError(
+                    "association proxy to a non-relationship "
+                    "intermediary is not supported"
+                ),
+                replace_context=None,
+            )
+
+        target_class = prop.mapper.class_
+
+        try:
+            target_assoc = cls._cls_unwrap_target_assoc_proxy(
+                target_class, value_attr
+            )
+        except AttributeError:
+            # the proxied attribute doesn't exist on the target class;
+            # return an "ambiguous" instance that will work on a per-object
+            # basis
+            return AmbiguousAssociationProxyInstance(
+                parent, owning_class, target_class, value_attr
+            )
+        except Exception as err:
+            util.raise_(
+                exc.InvalidRequestError(
+                    "Association proxy received an unexpected error when "
+                    "trying to retreive attribute "
+                    '"%s.%s" from '
+                    'class "%s": %s'
+                    % (
+                        target_class.__name__,
+                        parent.value_attr,
+                        target_class.__name__,
+                        err,
+                    )
+                ),
+                from_=err,
+            )
+        else:
+            return cls._construct_for_assoc(
+                target_assoc, parent, owning_class, target_class, value_attr
+            )
+
+    @classmethod
+    def _construct_for_assoc(
+        cls, target_assoc, parent, owning_class, target_class, value_attr
+    ):
+        if target_assoc is not None:
+            return ObjectAssociationProxyInstance(
+                parent, owning_class, target_class, value_attr
+            )
+
+        attr = getattr(target_class, value_attr)
+        if not hasattr(attr, "_is_internal_proxy"):
+            return AmbiguousAssociationProxyInstance(
+                parent, owning_class, target_class, value_attr
+            )
+        is_object = attr._impl_uses_objects
+        if is_object:
+            return ObjectAssociationProxyInstance(
+                parent, owning_class, target_class, value_attr
+            )
+        else:
+            return ColumnAssociationProxyInstance(
+                parent, owning_class, target_class, value_attr
+            )
+
+    def _get_property(self):
+        return orm.class_mapper(self.owning_class).get_property(
+            self.target_collection
+        )
+
+    @property
+    def _comparator(self):
+        return self._get_property().comparator
+
+    def __clause_element__(self):
+        raise NotImplementedError(
+            "The association proxy can't be used as a plain column "
+            "expression; it only works inside of a comparison expression"
+        )
+
+    @classmethod
+    def _cls_unwrap_target_assoc_proxy(cls, target_class, value_attr):
+        attr = getattr(target_class, value_attr)
+        if isinstance(attr, (AssociationProxy, AssociationProxyInstance)):
+            return attr
+        return None
+
+    @util.memoized_property
+    def _unwrap_target_assoc_proxy(self):
+        return self._cls_unwrap_target_assoc_proxy(
+            self.target_class, self.value_attr
+        )
+
+    @property
+    def remote_attr(self):
+        """The 'remote' class attribute referenced by this
+        :class:`.AssociationProxyInstance`.
+
+        .. seealso::
+
+            :attr:`.AssociationProxyInstance.attr`
+
+            :attr:`.AssociationProxyInstance.local_attr`
+
+        """
+        return getattr(self.target_class, self.value_attr)
+
+    @property
+    def local_attr(self):
+        """The 'local' class attribute referenced by this
+        :class:`.AssociationProxyInstance`.
+
+        .. seealso::
+
+            :attr:`.AssociationProxyInstance.attr`
+
+            :attr:`.AssociationProxyInstance.remote_attr`
+
+        """
+        return getattr(self.owning_class, self.target_collection)
+
+    @property
+    def attr(self):
+        """Return a tuple of ``(local_attr, remote_attr)``.
+
+        This attribute was originally intended to facilitate using the
+        :meth:`_query.Query.join` method to join across the two relationships
+        at once, however this makes use of a deprecated calling style.
+
+        To use :meth:`_sql.select.join` or :meth:`_orm.Query.join` with
+        an association proxy, the current method is to make use of the
+        :attr:`.AssociationProxyInstance.local_attr` and
+        :attr:`.AssociationProxyInstance.remote_attr` attributes separately::
+
+            stmt = (
+                select(Parent).
+                join(Parent.proxied.local_attr).
+                join(Parent.proxied.remote_attr)
+            )
+
+        A future release may seek to provide a more succinct join pattern
+        for association proxy attributes.
+
+        .. seealso::
+
+            :attr:`.AssociationProxyInstance.local_attr`
+
+            :attr:`.AssociationProxyInstance.remote_attr`
+
+        """
+        return (self.local_attr, self.remote_attr)
+
+    @util.memoized_property
+    def scalar(self):
+        """Return ``True`` if this :class:`.AssociationProxyInstance`
+        proxies a scalar relationship on the local side."""
+
+        scalar = not self._get_property().uselist
+        if scalar:
+            self._initialize_scalar_accessors()
+        return scalar
+
+    @util.memoized_property
+    def _value_is_scalar(self):
+        return (
+            not self._get_property()
+            .mapper.get_property(self.value_attr)
+            .uselist
+        )
+
+    @property
+    def _target_is_object(self):
+        raise NotImplementedError()
+
+    def _initialize_scalar_accessors(self):
+        if self.parent.getset_factory:
+            get, set_ = self.parent.getset_factory(None, self)
+        else:
+            get, set_ = self.parent._default_getset(None)
+        self._scalar_get, self._scalar_set = get, set_
+
+    def _default_getset(self, collection_class):
+        attr = self.value_attr
+        _getter = operator.attrgetter(attr)
+
+        def getter(target):
+            return _getter(target) if target is not None else None
+
+        if collection_class is dict:
+
+            def setter(o, k, v):
+                return setattr(o, attr, v)
+
+        else:
+
+            def setter(o, v):
+                return setattr(o, attr, v)
+
+        return getter, setter
+
+    @property
+    def info(self):
+        return self.parent.info
+
+    def get(self, obj):
+        if obj is None:
+            return self
+
+        if self.scalar:
+            target = getattr(obj, self.target_collection)
+            return self._scalar_get(target)
+        else:
+            try:
+                # If the owning instance is reborn (orm session resurrect,
+                # etc.), refresh the proxy cache.
+                creator_id, self_id, proxy = getattr(obj, self.key)
+            except AttributeError:
+                pass
+            else:
+                if id(obj) == creator_id and id(self) == self_id:
+                    assert self.collection_class is not None
+                    return proxy
+
+            self.collection_class, proxy = self._new(
+                _lazy_collection(obj, self.target_collection)
+            )
+            setattr(obj, self.key, (id(obj), id(self), proxy))
+            return proxy
+
+    def set(self, obj, values):
+        if self.scalar:
+            creator = (
+                self.parent.creator
+                if self.parent.creator
+                else self.target_class
+            )
+            target = getattr(obj, self.target_collection)
+            if target is None:
+                if values is None:
+                    return
+                setattr(obj, self.target_collection, creator(values))
+            else:
+                self._scalar_set(target, values)
+                if values is None and self.parent.cascade_scalar_deletes:
+                    setattr(obj, self.target_collection, None)
+        else:
+            proxy = self.get(obj)
+            assert self.collection_class is not None
+            if proxy is not values:
+                proxy._bulk_replace(self, values)
+
+    def delete(self, obj):
+        if self.owning_class is None:
+            self._calc_owner(obj, None)
+
+        if self.scalar:
+            target = getattr(obj, self.target_collection)
+            if target is not None:
+                delattr(target, self.value_attr)
+        delattr(obj, self.target_collection)
+
+    def _new(self, lazy_collection):
+        creator = (
+            self.parent.creator if self.parent.creator else self.target_class
+        )
+        collection_class = util.duck_type_collection(lazy_collection())
+
+        if self.parent.proxy_factory:
+            return (
+                collection_class,
+                self.parent.proxy_factory(
+                    lazy_collection, creator, self.value_attr, self
+                ),
+            )
+
+        if self.parent.getset_factory:
+            getter, setter = self.parent.getset_factory(collection_class, self)
+        else:
+            getter, setter = self.parent._default_getset(collection_class)
+
+        if collection_class is list:
+            return (
+                collection_class,
+                _AssociationList(
+                    lazy_collection, creator, getter, setter, self
+                ),
+            )
+        elif collection_class is dict:
+            return (
+                collection_class,
+                _AssociationDict(
+                    lazy_collection, creator, getter, setter, self
+                ),
+            )
+        elif collection_class is set:
+            return (
+                collection_class,
+                _AssociationSet(
+                    lazy_collection, creator, getter, setter, self
+                ),
+            )
+        else:
+            raise exc.ArgumentError(
+                "could not guess which interface to use for "
+                'collection_class "%s" backing "%s"; specify a '
+                "proxy_factory and proxy_bulk_set manually"
+                % (self.collection_class.__name__, self.target_collection)
+            )
+
+    def _set(self, proxy, values):
+        if self.parent.proxy_bulk_set:
+            self.parent.proxy_bulk_set(proxy, values)
+        elif self.collection_class is list:
+            proxy.extend(values)
+        elif self.collection_class is dict:
+            proxy.update(values)
+        elif self.collection_class is set:
+            proxy.update(values)
+        else:
+            raise exc.ArgumentError(
+                "no proxy_bulk_set supplied for custom "
+                "collection_class implementation"
+            )
+
+    def _inflate(self, proxy):
+        creator = (
+            self.parent.creator and self.parent.creator or self.target_class
+        )
+
+        if self.parent.getset_factory:
+            getter, setter = self.parent.getset_factory(
+                self.collection_class, self
+            )
+        else:
+            getter, setter = self.parent._default_getset(self.collection_class)
+
+        proxy.creator = creator
+        proxy.getter = getter
+        proxy.setter = setter
+
+    def _criterion_exists(self, criterion=None, **kwargs):
+        is_has = kwargs.pop("is_has", None)
+
+        target_assoc = self._unwrap_target_assoc_proxy
+        if target_assoc is not None:
+            inner = target_assoc._criterion_exists(
+                criterion=criterion, **kwargs
+            )
+            return self._comparator._criterion_exists(inner)
+
+        if self._target_is_object:
+            prop = getattr(self.target_class, self.value_attr)
+            value_expr = prop._criterion_exists(criterion, **kwargs)
+        else:
+            if kwargs:
+                raise exc.ArgumentError(
+                    "Can't apply keyword arguments to column-targeted "
+                    "association proxy; use =="
+                )
+            elif is_has and criterion is not None:
+                raise exc.ArgumentError(
+                    "Non-empty has() not allowed for "
+                    "column-targeted association proxy; use =="
+                )
+
+            value_expr = criterion
+
+        return self._comparator._criterion_exists(value_expr)
+
+    def any(self, criterion=None, **kwargs):
+        """Produce a proxied 'any' expression using EXISTS.
+
+        This expression will be a composed product
+        using the :meth:`.RelationshipProperty.Comparator.any`
+        and/or :meth:`.RelationshipProperty.Comparator.has`
+        operators of the underlying proxied attributes.
+
+        """
+        if self._unwrap_target_assoc_proxy is None and (
+            self.scalar
+            and (not self._target_is_object or self._value_is_scalar)
+        ):
+            raise exc.InvalidRequestError(
+                "'any()' not implemented for scalar " "attributes. Use has()."
+            )
+        return self._criterion_exists(
+            criterion=criterion, is_has=False, **kwargs
+        )
+
+    def has(self, criterion=None, **kwargs):
+        """Produce a proxied 'has' expression using EXISTS.
+
+        This expression will be a composed product
+        using the :meth:`.RelationshipProperty.Comparator.any`
+        and/or :meth:`.RelationshipProperty.Comparator.has`
+        operators of the underlying proxied attributes.
+
+        """
+        if self._unwrap_target_assoc_proxy is None and (
+            not self.scalar
+            or (self._target_is_object and not self._value_is_scalar)
+        ):
+            raise exc.InvalidRequestError(
+                "'has()' not implemented for collections.  " "Use any()."
+            )
+        return self._criterion_exists(
+            criterion=criterion, is_has=True, **kwargs
+        )
+
+    def __repr__(self):
+        return "%s(%r)" % (self.__class__.__name__, self.parent)
+
+
+class AmbiguousAssociationProxyInstance(AssociationProxyInstance):
+    """an :class:`.AssociationProxyInstance` where we cannot determine
+    the type of target object.
+    """
+
+    _is_canonical = False
+
+    def _ambiguous(self):
+        raise AttributeError(
+            "Association proxy %s.%s refers to an attribute '%s' that is not "
+            "directly mapped on class %s; therefore this operation cannot "
+            "proceed since we don't know what type of object is referred "
+            "towards"
+            % (
+                self.owning_class.__name__,
+                self.target_collection,
+                self.value_attr,
+                self.target_class,
+            )
+        )
+
+    def get(self, obj):
+        if obj is None:
+            return self
+        else:
+            return super(AmbiguousAssociationProxyInstance, self).get(obj)
+
+    def __eq__(self, obj):
+        self._ambiguous()
+
+    def __ne__(self, obj):
+        self._ambiguous()
+
+    def any(self, criterion=None, **kwargs):
+        self._ambiguous()
+
+    def has(self, criterion=None, **kwargs):
+        self._ambiguous()
+
+    @util.memoized_property
+    def _lookup_cache(self):
+        # mapping of <subclass>->AssociationProxyInstance.
+        # e.g. proxy is A-> A.b -> B -> B.b_attr, but B.b_attr doesn't exist;
+        # only B1(B) and B2(B) have "b_attr", keys in here would be B1, B2
+        return {}
+
+    def _non_canonical_get_for_object(self, parent_instance):
+        if parent_instance is not None:
+            actual_obj = getattr(parent_instance, self.target_collection)
+            if actual_obj is not None:
+                try:
+                    insp = inspect(actual_obj)
+                except exc.NoInspectionAvailable:
+                    pass
+                else:
+                    mapper = insp.mapper
+                    instance_class = mapper.class_
+                    if instance_class not in self._lookup_cache:
+                        self._populate_cache(instance_class, mapper)
+
+                    try:
+                        return self._lookup_cache[instance_class]
+                    except KeyError:
+                        pass
+
+        # no object or ambiguous object given, so return "self", which
+        # is a proxy with generally only instance-level functionality
+        return self
+
+    def _populate_cache(self, instance_class, mapper):
+        prop = orm.class_mapper(self.owning_class).get_property(
+            self.target_collection
+        )
+
+        if mapper.isa(prop.mapper):
+            target_class = instance_class
+            try:
+                target_assoc = self._cls_unwrap_target_assoc_proxy(
+                    target_class, self.value_attr
+                )
+            except AttributeError:
+                pass
+            else:
+                self._lookup_cache[instance_class] = self._construct_for_assoc(
+                    target_assoc,
+                    self.parent,
+                    self.owning_class,
+                    target_class,
+                    self.value_attr,
+                )
+
+
+class ObjectAssociationProxyInstance(AssociationProxyInstance):
+    """an :class:`.AssociationProxyInstance` that has an object as a target."""
+
+    _target_is_object = True
+    _is_canonical = True
+
+    def contains(self, obj):
+        """Produce a proxied 'contains' expression using EXISTS.
+
+        This expression will be a composed product
+        using the :meth:`.RelationshipProperty.Comparator.any`,
+        :meth:`.RelationshipProperty.Comparator.has`,
+        and/or :meth:`.RelationshipProperty.Comparator.contains`
+        operators of the underlying proxied attributes.
+        """
+
+        target_assoc = self._unwrap_target_assoc_proxy
+        if target_assoc is not None:
+            return self._comparator._criterion_exists(
+                target_assoc.contains(obj)
+                if not target_assoc.scalar
+                else target_assoc == obj
+            )
+        elif (
+            self._target_is_object
+            and self.scalar
+            and not self._value_is_scalar
+        ):
+            return self._comparator.has(
+                getattr(self.target_class, self.value_attr).contains(obj)
+            )
+        elif self._target_is_object and self.scalar and self._value_is_scalar:
+            raise exc.InvalidRequestError(
+                "contains() doesn't apply to a scalar object endpoint; use =="
+            )
+        else:
+
+            return self._comparator._criterion_exists(**{self.value_attr: obj})
+
+    def __eq__(self, obj):
+        # note the has() here will fail for collections; eq_()
+        # is only allowed with a scalar.
+        if obj is None:
+            return or_(
+                self._comparator.has(**{self.value_attr: obj}),
+                self._comparator == None,
+            )
+        else:
+            return self._comparator.has(**{self.value_attr: obj})
+
+    def __ne__(self, obj):
+        # note the has() here will fail for collections; eq_()
+        # is only allowed with a scalar.
+        return self._comparator.has(
+            getattr(self.target_class, self.value_attr) != obj
+        )
+
+
+class ColumnAssociationProxyInstance(
+    ColumnOperators, AssociationProxyInstance
+):
+    """an :class:`.AssociationProxyInstance` that has a database column as a
+    target.
+    """
+
+    _target_is_object = False
+    _is_canonical = True
+
+    def __eq__(self, other):
+        # special case "is None" to check for no related row as well
+        expr = self._criterion_exists(
+            self.remote_attr.operate(operator.eq, other)
+        )
+        if other is None:
+            return or_(expr, self._comparator == None)
+        else:
+            return expr
+
+    def operate(self, op, *other, **kwargs):
+        return self._criterion_exists(
+            self.remote_attr.operate(op, *other, **kwargs)
+        )
+
+
+class _lazy_collection(object):
+    def __init__(self, obj, target):
+        self.parent = obj
+        self.target = target
+
+    def __call__(self):
+        return getattr(self.parent, self.target)
+
+    def __getstate__(self):
+        return {"obj": self.parent, "target": self.target}
+
+    def __setstate__(self, state):
+        self.parent = state["obj"]
+        self.target = state["target"]
+
+
+class _AssociationCollection(object):
+    def __init__(self, lazy_collection, creator, getter, setter, parent):
+        """Constructs an _AssociationCollection.
+
+        This will always be a subclass of either _AssociationList,
+        _AssociationSet, or _AssociationDict.
+
+        lazy_collection
+          A callable returning a list-based collection of entities (usually an
+          object attribute managed by a SQLAlchemy relationship())
+
+        creator
+          A function that creates new target entities.  Given one parameter:
+          value.  This assertion is assumed::
+
+            obj = creator(somevalue)
+            assert getter(obj) == somevalue
+
+        getter
+          A function.  Given an associated object, return the 'value'.
+
+        setter
+          A function.  Given an associated object and a value, store that
+          value on the object.
+
+        """
+        self.lazy_collection = lazy_collection
+        self.creator = creator
+        self.getter = getter
+        self.setter = setter
+        self.parent = parent
+
+    col = property(lambda self: self.lazy_collection())
+
+    def __len__(self):
+        return len(self.col)
+
+    def __bool__(self):
+        return bool(self.col)
+
+    __nonzero__ = __bool__
+
+    def __getstate__(self):
+        return {"parent": self.parent, "lazy_collection": self.lazy_collection}
+
+    def __setstate__(self, state):
+        self.parent = state["parent"]
+        self.lazy_collection = state["lazy_collection"]
+        self.parent._inflate(self)
+
+    def _bulk_replace(self, assoc_proxy, values):
+        self.clear()
+        assoc_proxy._set(self, values)
+
+
+class _AssociationList(_AssociationCollection):
+    """Generic, converting, list-to-list proxy."""
+
+    def _create(self, value):
+        return self.creator(value)
+
+    def _get(self, object_):
+        return self.getter(object_)
+
+    def _set(self, object_, value):
+        return self.setter(object_, value)
+
+    def __getitem__(self, index):
+        if not isinstance(index, slice):
+            return self._get(self.col[index])
+        else:
+            return [self._get(member) for member in self.col[index]]
+
+    def __setitem__(self, index, value):
+        if not isinstance(index, slice):
+            self._set(self.col[index], value)
+        else:
+            if index.stop is None:
+                stop = len(self)
+            elif index.stop < 0:
+                stop = len(self) + index.stop
+            else:
+                stop = index.stop
+            step = index.step or 1
+
+            start = index.start or 0
+            rng = list(range(index.start or 0, stop, step))
+            if step == 1:
+                for i in rng:
+                    del self[start]
+                i = start
+                for item in value:
+                    self.insert(i, item)
+                    i += 1
+            else:
+                if len(value) != len(rng):
+                    raise ValueError(
+                        "attempt to assign sequence of size %s to "
+                        "extended slice of size %s" % (len(value), len(rng))
+                    )
+                for i, item in zip(rng, value):
+                    self._set(self.col[i], item)
+
+    def __delitem__(self, index):
+        del self.col[index]
+
+    def __contains__(self, value):
+        for member in self.col:
+            # testlib.pragma exempt:__eq__
+            if self._get(member) == value:
+                return True
+        return False
+
+    def __getslice__(self, start, end):
+        return [self._get(member) for member in self.col[start:end]]
+
+    def __setslice__(self, start, end, values):
+        members = [self._create(v) for v in values]
+        self.col[start:end] = members
+
+    def __delslice__(self, start, end):
+        del self.col[start:end]
+
+    def __iter__(self):
+        """Iterate over proxied values.
+
+        For the actual domain objects, iterate over .col instead or
+        just use the underlying collection directly from its property
+        on the parent.
+        """
+
+        for member in self.col:
+            yield self._get(member)
+        return
+
+    def append(self, value):
+        col = self.col
+        item = self._create(value)
+        col.append(item)
+
+    def count(self, value):
+        return sum(
+            [
+                1
+                for _ in util.itertools_filter(
+                    lambda v: v == value, iter(self)
+                )
+            ]
+        )
+
+    def extend(self, values):
+        for v in values:
+            self.append(v)
+
+    def insert(self, index, value):
+        self.col[index:index] = [self._create(value)]
+
+    def pop(self, index=-1):
+        return self.getter(self.col.pop(index))
+
+    def remove(self, value):
+        for i, val in enumerate(self):
+            if val == value:
+                del self.col[i]
+                return
+        raise ValueError("value not in list")
+
+    def reverse(self):
+        """Not supported, use reversed(mylist)"""
+
+        raise NotImplementedError
+
+    def sort(self):
+        """Not supported, use sorted(mylist)"""
+
+        raise NotImplementedError
+
+    def clear(self):
+        del self.col[0 : len(self.col)]
+
+    def __eq__(self, other):
+        return list(self) == other
+
+    def __ne__(self, other):
+        return list(self) != other
+
+    def __lt__(self, other):
+        return list(self) < other
+
+    def __le__(self, other):
+        return list(self) <= other
+
+    def __gt__(self, other):
+        return list(self) > other
+
+    def __ge__(self, other):
+        return list(self) >= other
+
+    def __cmp__(self, other):
+        return util.cmp(list(self), other)
+
+    def __add__(self, iterable):
+        try:
+            other = list(iterable)
+        except TypeError:
+            return NotImplemented
+        return list(self) + other
+
+    def __radd__(self, iterable):
+        try:
+            other = list(iterable)
+        except TypeError:
+            return NotImplemented
+        return other + list(self)
+
+    def __mul__(self, n):
+        if not isinstance(n, int):
+            return NotImplemented
+        return list(self) * n
+
+    __rmul__ = __mul__
+
+    def __iadd__(self, iterable):
+        self.extend(iterable)
+        return self
+
+    def __imul__(self, n):
+        # unlike a regular list *=, proxied __imul__ will generate unique
+        # backing objects for each copy.  *= on proxied lists is a bit of
+        # a stretch anyhow, and this interpretation of the __imul__ contract
+        # is more plausibly useful than copying the backing objects.
+        if not isinstance(n, int):
+            return NotImplemented
+        if n == 0:
+            self.clear()
+        elif n > 1:
+            self.extend(list(self) * (n - 1))
+        return self
+
+    def index(self, item, *args):
+        return list(self).index(item, *args)
+
+    def copy(self):
+        return list(self)
+
+    def __repr__(self):
+        return repr(list(self))
+
+    def __hash__(self):
+        raise TypeError("%s objects are unhashable" % type(self).__name__)
+
+    for func_name, func in list(locals().items()):
+        if (
+            callable(func)
+            and func.__name__ == func_name
+            and not func.__doc__
+            and hasattr(list, func_name)
+        ):
+            func.__doc__ = getattr(list, func_name).__doc__
+    del func_name, func
+
+
+_NotProvided = util.symbol("_NotProvided")
+
+
+class _AssociationDict(_AssociationCollection):
+    """Generic, converting, dict-to-dict proxy."""
+
+    def _create(self, key, value):
+        return self.creator(key, value)
+
+    def _get(self, object_):
+        return self.getter(object_)
+
+    def _set(self, object_, key, value):
+        return self.setter(object_, key, value)
+
+    def __getitem__(self, key):
+        return self._get(self.col[key])
+
+    def __setitem__(self, key, value):
+        if key in self.col:
+            self._set(self.col[key], key, value)
+        else:
+            self.col[key] = self._create(key, value)
+
+    def __delitem__(self, key):
+        del self.col[key]
+
+    def __contains__(self, key):
+        # testlib.pragma exempt:__hash__
+        return key in self.col
+
+    def has_key(self, key):
+        # testlib.pragma exempt:__hash__
+        return key in self.col
+
+    def __iter__(self):
+        return iter(self.col.keys())
+
+    def clear(self):
+        self.col.clear()
+
+    def __eq__(self, other):
+        return dict(self) == other
+
+    def __ne__(self, other):
+        return dict(self) != other
+
+    def __lt__(self, other):
+        return dict(self) < other
+
+    def __le__(self, other):
+        return dict(self) <= other
+
+    def __gt__(self, other):
+        return dict(self) > other
+
+    def __ge__(self, other):
+        return dict(self) >= other
+
+    def __cmp__(self, other):
+        return util.cmp(dict(self), other)
+
+    def __repr__(self):
+        return repr(dict(self.items()))
+
+    def get(self, key, default=None):
+        try:
+            return self[key]
+        except KeyError:
+            return default
+
+    def setdefault(self, key, default=None):
+        if key not in self.col:
+            self.col[key] = self._create(key, default)
+            return default
+        else:
+            return self[key]
+
+    def keys(self):
+        return self.col.keys()
+
+    if util.py2k:
+
+        def iteritems(self):
+            return ((key, self._get(self.col[key])) for key in self.col)
+
+        def itervalues(self):
+            return (self._get(self.col[key]) for key in self.col)
+
+        def iterkeys(self):
+            return self.col.iterkeys()
+
+        def values(self):
+            return [self._get(member) for member in self.col.values()]
+
+        def items(self):
+            return [(k, self._get(self.col[k])) for k in self]
+
+    else:
+
+        def items(self):
+            return ((key, self._get(self.col[key])) for key in self.col)
+
+        def values(self):
+            return (self._get(self.col[key]) for key in self.col)
+
+    def pop(self, key, default=_NotProvided):
+        if default is _NotProvided:
+            member = self.col.pop(key)
+        else:
+            member = self.col.pop(key, default)
+        return self._get(member)
+
+    def popitem(self):
+        item = self.col.popitem()
+        return (item[0], self._get(item[1]))
+
+    def update(self, *a, **kw):
+        if len(a) > 1:
+            raise TypeError(
+                "update expected at most 1 arguments, got %i" % len(a)
+            )
+        elif len(a) == 1:
+            seq_or_map = a[0]
+            # discern dict from sequence - took the advice from
+            # https://www.voidspace.org.uk/python/articles/duck_typing.shtml
+            # still not perfect :(
+            if hasattr(seq_or_map, "keys"):
+                for item in seq_or_map:
+                    self[item] = seq_or_map[item]
+            else:
+                try:
+                    for k, v in seq_or_map:
+                        self[k] = v
+                except ValueError as err:
+                    util.raise_(
+                        ValueError(
+                            "dictionary update sequence "
+                            "requires 2-element tuples"
+                        ),
+                        replace_context=err,
+                    )
+
+        for key, value in kw:
+            self[key] = value
+
+    def _bulk_replace(self, assoc_proxy, values):
+        existing = set(self)
+        constants = existing.intersection(values or ())
+        additions = set(values or ()).difference(constants)
+        removals = existing.difference(constants)
+
+        for key, member in values.items() or ():
+            if key in additions:
+                self[key] = member
+            elif key in constants:
+                self[key] = member
+
+        for key in removals:
+            del self[key]
+
+    def copy(self):
+        return dict(self.items())
+
+    def __hash__(self):
+        raise TypeError("%s objects are unhashable" % type(self).__name__)
+
+    for func_name, func in list(locals().items()):
+        if (
+            callable(func)
+            and func.__name__ == func_name
+            and not func.__doc__
+            and hasattr(dict, func_name)
+        ):
+            func.__doc__ = getattr(dict, func_name).__doc__
+    del func_name, func
+
+
+class _AssociationSet(_AssociationCollection):
+    """Generic, converting, set-to-set proxy."""
+
+    def _create(self, value):
+        return self.creator(value)
+
+    def _get(self, object_):
+        return self.getter(object_)
+
+    def __len__(self):
+        return len(self.col)
+
+    def __bool__(self):
+        if self.col:
+            return True
+        else:
+            return False
+
+    __nonzero__ = __bool__
+
+    def __contains__(self, value):
+        for member in self.col:
+            # testlib.pragma exempt:__eq__
+            if self._get(member) == value:
+                return True
+        return False
+
+    def __iter__(self):
+        """Iterate over proxied values.
+
+        For the actual domain objects, iterate over .col instead or just use
+        the underlying collection directly from its property on the parent.
+
+        """
+        for member in self.col:
+            yield self._get(member)
+        return
+
+    def add(self, value):
+        if value not in self:
+            self.col.add(self._create(value))
+
+    # for discard and remove, choosing a more expensive check strategy rather
+    # than call self.creator()
+    def discard(self, value):
+        for member in self.col:
+            if self._get(member) == value:
+                self.col.discard(member)
+                break
+
+    def remove(self, value):
+        for member in self.col:
+            if self._get(member) == value:
+                self.col.discard(member)
+                return
+        raise KeyError(value)
+
+    def pop(self):
+        if not self.col:
+            raise KeyError("pop from an empty set")
+        member = self.col.pop()
+        return self._get(member)
+
+    def update(self, other):
+        for value in other:
+            self.add(value)
+
+    def _bulk_replace(self, assoc_proxy, values):
+        existing = set(self)
+        constants = existing.intersection(values or ())
+        additions = set(values or ()).difference(constants)
+        removals = existing.difference(constants)
+
+        appender = self.add
+        remover = self.remove
+
+        for member in values or ():
+            if member in additions:
+                appender(member)
+            elif member in constants:
+                appender(member)
+
+        for member in removals:
+            remover(member)
+
+    def __ior__(self, other):
+        if not collections._set_binops_check_strict(self, other):
+            return NotImplemented
+        for value in other:
+            self.add(value)
+        return self
+
+    def _set(self):
+        return set(iter(self))
+
+    def union(self, other):
+        return set(self).union(other)
+
+    __or__ = union
+
+    def difference(self, other):
+        return set(self).difference(other)
+
+    __sub__ = difference
+
+    def difference_update(self, other):
+        for value in other:
+            self.discard(value)
+
+    def __isub__(self, other):
+        if not collections._set_binops_check_strict(self, other):
+            return NotImplemented
+        for value in other:
+            self.discard(value)
+        return self
+
+    def intersection(self, other):
+        return set(self).intersection(other)
+
+    __and__ = intersection
+
+    def intersection_update(self, other):
+        want, have = self.intersection(other), set(self)
+
+        remove, add = have - want, want - have
+
+        for value in remove:
+            self.remove(value)
+        for value in add:
+            self.add(value)
+
+    def __iand__(self, other):
+        if not collections._set_binops_check_strict(self, other):
+            return NotImplemented
+        want, have = self.intersection(other), set(self)
+
+        remove, add = have - want, want - have
+
+        for value in remove:
+            self.remove(value)
+        for value in add:
+            self.add(value)
+        return self
+
+    def symmetric_difference(self, other):
+        return set(self).symmetric_difference(other)
+
+    __xor__ = symmetric_difference
+
+    def symmetric_difference_update(self, other):
+        want, have = self.symmetric_difference(other), set(self)
+
+        remove, add = have - want, want - have
+
+        for value in remove:
+            self.remove(value)
+        for value in add:
+            self.add(value)
+
+    def __ixor__(self, other):
+        if not collections._set_binops_check_strict(self, other):
+            return NotImplemented
+        want, have = self.symmetric_difference(other), set(self)
+
+        remove, add = have - want, want - have
+
+        for value in remove:
+            self.remove(value)
+        for value in add:
+            self.add(value)
+        return self
+
+    def issubset(self, other):
+        return set(self).issubset(other)
+
+    def issuperset(self, other):
+        return set(self).issuperset(other)
+
+    def clear(self):
+        self.col.clear()
+
+    def copy(self):
+        return set(self)
+
+    def __eq__(self, other):
+        return set(self) == other
+
+    def __ne__(self, other):
+        return set(self) != other
+
+    def __lt__(self, other):
+        return set(self) < other
+
+    def __le__(self, other):
+        return set(self) <= other
+
+    def __gt__(self, other):
+        return set(self) > other
+
+    def __ge__(self, other):
+        return set(self) >= other
+
+    def __repr__(self):
+        return repr(set(self))
+
+    def __hash__(self):
+        raise TypeError("%s objects are unhashable" % type(self).__name__)
+
+    for func_name, func in list(locals().items()):
+        if (
+            callable(func)
+            and func.__name__ == func_name
+            and not func.__doc__
+            and hasattr(set, func_name)
+        ):
+            func.__doc__ = getattr(set, func_name).__doc__
+    del func_name, func