- put an aggressive check for "flushing object A with a collection

- put an aggressive check for "flushing object A with a collection
of B's, but you put a C in the collection" error condition -
**even if C is a subclass of B**, unless B's mapper loads polymorphically.
Otherwise, the collection will later load a "B" which should be a "C"
(since its not polymorphic) which breaks in bi-directional relationships
(i.e. C has its A, but A's backref will lazyload it as a different
instance of type "B") [ticket:500]