test_model.py

def test_colection_uniqueness(self):
    collection = CollectionFactory()
    collection2 = CollectionFactory()

    collection.site = 'foo'
    collection.slug = 'foo'

    collection2.site = 'foo'
    collection2.site = 'foo'

    collection.save()
    with self.assertRaises(ValidationError):
        collection2.save()

    collection2.site = 'bar'
    # No longer throws a Validation error.
    collection2.slug = 'bar'