The :meth:`.AssociationProxy.any`, :meth:`.AssociationProxy.has`
and :meth:`.AssociationProxy.contains`
comparison methods now support linkage to an attribute that
is itself also an :class:`.AssociationProxy`, recursively.

After some initial attempts it's clear that the any() / has()
of AssociationProxy needed to be reworked into a generic
_criterion_exists() to allow this to work recursively without
excess complexity.  For the case of the multi-linked associationproxy,
the usual checks of "any()" / "has()" correctness simply don't
take place; for a single-link association proxy the error
checking logic that takes place in relationship() has been
ported to the local any() / has() methods.

Change-Id: Ic5aed2a4e910b8138a737d215430113c31cce856
Fixes: #3769

A warning is emitted if the :attr:`.declared_attr.cascading` modifier
is used with a declarative attribute that is itself declared on
a class that is to be mapped, as opposed to a declarative mixin
class or ``__abstract__`` class.  The :attr:`.declared_attr.cascading`
modifier currently only applies to mixin/abstract classes.

Also add a test for @declared_attr.cascading when used on an attribute
on __abstract__.

Change-Id: Ib1b9dbe373e8be1cf24eadfed224a8988b3cd95d
Fixes: #3847

Implemented in-place mutation operators ``__ior__``, ``__iand__``,
``__ixor__`` and ``__isub__`` for :class:`.mutable.MutableSet`
and ``__iadd__`` for :class:`.mutable.MutableList` so that change
events are fired off when these mutator methods are used to alter the
collection.

Change-Id: Ib357a96d3b06c5deb6b53eb304a8b9f1dc9e9ede
Fixes: #3853

Added a new style of mapper-level inheritance loading
"polymorphic selectin".  This style of loading
emits queries for each subclass in an inheritance
hierarchy subsequent to the load of the base
object type, using IN to specify the desired
primary key values.

Fixes: #3948

Change-Id: I59e071c6142354a3f95730046e3dcdfc0e2c4de5

Fixed bug where using :class:`.declared_attr` on an
:class:`.AbstractConcreteBase` where a particular return value were some
non-mapped symbol, including ``None``, would cause the attribute
to hard-evaluate just once and store the value to the object
dictionary, not allowing it to invoke for subclasses.   This behavior
is normal when :class:`.declared_attr` is on a mapped class, and
does not occur on a mixin or abstract class.  Since
:class:`.AbstractConcreteBase` is both "abstract" and actually
"mapped", a special exception case is made here so that the
"abstract" behavior takes precedence for :class:`.declared_attr`.

Change-Id: I6160ebb3a52c441d6a4b663c8c9bbac6d37fa417
Fixes: #3848

Added new event handler :meth:`.AttributeEvents.modified` which is
triggered when the func:`.attributes.flag_modified` function is
invoked, which is common when using the :mod:`sqlalchemy.ext.mutable`
extension module.

Change-Id: Ic152f1d5c53087d780b24ed7f1f1571527b9e8fc
Fixes: #3303

Added new flag :paramref:`.Session.enable_baked_queries` to the
:class:`.Session` to allow baked queries to be disabled
session-wide, reducing memory use.   Also added new :class:`.Bakery`
wrapper so that the bakery returned by :paramref:`.BakedQuery.bakery`
can be inspected.

Change-Id: I5657af7a99d2b24c89d6aee1343f432728e3f807

level cache
- include SelectinLoader itself in the cache key, though
this is currently not critical

Change-Id: I8e4bcd579277fbe53d9c7eca3552a0b8ab9d7a39

Protected against testing "None" as a class in the case where
declarative classes are being garbage collected and new
automap prepare() operations are taking place concurrently, very
infrequently hitting a weakref that has not been fully acted upon
after gc.

Change-Id: I32e1dfc5ac46bac4127fe808cfd18368e2fad9dd