class TestY(TestCase):

    def test_model_invalid_event(self):
        x1 = X.objects.create(name="test1")
        x2 = X(name="test2")
        y = Y(event=x2)
        with self.assertRaises(ValidationError):
            y.full_clean()
            y.save()
        self.assertEqual(Y.objects.filter(event__name="test2").count(), 0)