See #165.
When the currently cached obj's PK matches the new FK, it should not be uncached.
The current behavior interacts badly with the current implementation of MappedOneToMany.save, which sets some of its children's FK. Subsequently calling obj on the child will reload a new instance of its parent. Thus the child cannot be relied upon to remove itself from its parent (via OneToMany) because it will be pending deletion on the wrong parent instance.
Admittedly this is a corner case but the situation could be improved.