Periodically running into bugs with optional to-one relationships when
people forget to check to-dep-pk checkbox, and end up having
FaultFailureExceptions. The issue is as old as Cayenne (see for
instance CAY-132). They way I think we should fix it is by introducing
another method "ObjRelationship.isOptional()" which is logically not
the same as 'isSourceIndependentFromtTarget()', for use inside
I figured I'd post a note about this, considering how old the issue is.