def test_foreign_key_managers(self):
        marty = Person.objects.using('other').create(pk=1, name="Marty Alchin")
        Book.objects.using('other').create(pk=1, title="Pro Django",
                                           published=datetime.date(2008, 12, 16),
                                           editor=marty)

        self.assertEqual(marty.edited.db, 'other')
        self.assertEqual(marty.edited.db_manager('default').db, 'default')
        self.assertEqual(marty.edited.db_manager('default').all().db, 'default')
FK reverse relations are represented by managers, and can be controlled like managers
dataset/ETHPy150Open django/django/tests/multiple_database/tests.py/RouterTestCase.test_foreign_key_managers