Comment by RedCrowbar

If fetched as JSON: {"favories": [{"director": null, "author": "J.K Rowling"}, {"director": "Spielberg", "author": null}, ...]}

You can also query the actual object type by referring to the __type__ link:

  SELECT User {
    favorites: {
        __type__,
        [IS Movie].director,
        [IS Book].author,
    }
  };