def whose_dependent_on_who(self, state1, state2):
        """Given an object pair assuming `obj2` is a child of `obj1`,
        return a tuple with the dependent object second, or None if
        there is no dependency.
        """

        if state1 is state2:
            return None
        elif self.direction == ONETOMANY:
            return (state1, state2)
        else:
            return (state2, state1)