class X(models.Model):
    name = models.CharField(unique = True)

class Y(models.Model):
    event = models.ForeignKey(X)

class TestY(TestCase):
    x = X.objects.create(name="test1")
    x.save()
    y = Y(event=X.objects.create(name="test2"))
    with self.assertRaises(ValidationError):
        if y.full_clean()
            y.save()

    self.assert(0,Y.objects.filter(event__name="test2").count)