``cascade="all, delete-orphan"`` automatically on a one-to-many
relationship/backref where the foreign key is detected as containing
one or more non-nullable columns.  This argument is present in the
keywords passed to :func:`.automap.generate_relationship` in this
case and can still be overridden.  Additionally, if the
:class:`.ForeignKeyConstraint` specifies ``ondelete="CASCADE"``
for a non-nullable or ``ondelete="SET NULL"`` for a nullable set
of columns, the argument ``passive_deletes=True`` is also added to the
relationship.  Note that not all backends support reflection of
ondelete, but backends that do include Postgresql and MySQL.
fixes #3210

setups, where the attempt to check for "duplicate class name" in
declarative would hit upon a not-totally-cleaned-up weak reference
related to some other class being removed; the check here now ensures
the weakref still references an object before calling upon it further.
fixes #3208

thrown off during a collection replace event, if the
reorder_on_append flag were set to True.  The fix ensures that the
ordering list only impacts the list that is explicitly associated
with the object.
fixes #3191

If a class inherited from MutableDict (say, for instance, to add an update() method), coerce() would give back an instance of MutableDict instead of an instance of the derived class.

ref #3129