This kinda comes down to personal testing style, but I [detest](https://twitter.com/jspahrsummers/status/408848105430204416) mocking. I don't think it's representative of your API's behavior to consumers, and smells of poor decomposition if it's really necessary.

In this specific case, couldn't you just verify that `initialPhotoModel` returns the same thing as "photo model at the initial index?"