- Fixed a bug in the examples/generic_assocaitions/discriminator_on_association.py
example, where the subclasses of AddressAssociation were not being
mapped as "single table inheritance", leading to problems when trying
to use the mappings further.
- add some logging to path_registry to help debug eager loading
issues
- The :meth:`.PropComparator.of_type` modifier has been
improved in conjunction with loader directives such as
:func:`.joinedload` and :func:`.contains_eager` such that if
two :meth:`.PropComparator.of_type` modifiers of the same
base type/path are encountered, they will be joined together
into a single "polymorphic" entity, rather than replacing
the entity of type A with the one of type B.  E.g.
a joinedload of ``A.b.of_type(BSub1)->BSub1.c`` combined with
joinedload of ``A.b.of_type(BSub2)->BSub2.c`` will create a
single joinedload of ``A.b.of_type((BSub1, BSub2)) -> BSub1.c, BSub2.c``,
without the need for the ``with_polymorphic`` to be explicit
in the query.
fixes #3256
- The behavioral contract of the :attr:`.ForeignKeyConstraint.columns`
collection has been made consistent; this attribute is now a
:class:`.ColumnCollection` like that of all other constraints and
is initialized at the point when the constraint is associated with
a :class:`.Table`.
fixes #3243