Ticket #7908: note1.py

            # the following can replace line 605 of related.py
            # but this is not a proper fix...
            to_field = to_field
            if to_field == None:
                if to._meta.pk is None:
                    raise ValueError("The %s model does not have a primary key and therefore a ForeignKey cannot relate to it." % to_name)
                to_field = to._meta.pk.name