Backbone.js: Collection containing multiple Models with same ID

Question

I've a merged collection in Backbone which contains photos and albums.

To distinguish between them, i've added a field type which is either photo or album. When I populate the collection, I create different models within the Collection#model method

  model: (attrs, options) ->
    switch attrs.type
      when 'album' then new App.Models.Album(attrs, options)
      when 'photo' then new App.Models.Photo(attrs, options)

Now I've discoverd a strange bug where adding a photo and an album with the same ID (let's say 2) results in a merge.

I've tracked this down to these LOC in the source code. It seems that it's undoable without creating a fork of Backbone itself. I've tried it but it also fails 35 tests.

I thought of 4 different ways of doing this, I don't know which of them is the better one:

1. I could add a prefix to the id. Let's say photo_2. This causes a change in the backend as well as some changes in the frontend to don't hit the server at /photos/photo_2
2. I could fork Backbone and change these LOC.
3. I could create two separate collections but have to deal with a merge and a sort in the view (which effects clients performance and requires a rewriting of the backend)
4. I could start with a photo ID of, let's say 1000000. This would extremely decrease the probability that a given user which has uploaded a photo with a given ID has also created an album with the same ID.