Backbone js collection of collections issue

I'm running into an isssue when I try to create a collection of collections using backbone js. Here is the code :

Models and Collections :

var Track = Backbone.Model.extend({

    defaults : {
      title : ""
    }
})

var TrackCollection = Backbone.Collection.extend({

    model : Track,
})

var Playlist = Backbone.Model.extend({

    defaults : {
        name : "",
        tracks : new TrackCollection,
    }
})

var PlaylistCollection = Backbone.Collection.extend({

    model : Playlist,
})

Creation of the playlist collection :

var playlists = new PlaylistCollection;

// create and push the first playlist
playlists.push({ name : "classic" });
// create and push a track in the playlist just created
playlists.last().get("tracks").push({ title : "fur elise" });

// create and push the second playlist
playlists.push({ name : "c2c" });
// create and push a track in the playlist just created
playlists.last().get("tracks").push({ title : "fuya" });

// display first playlist
console.log(JSON.stringify(playlists.at(0).toJSON()))
// display second playlist
console.log(JSON.stringify(playlists.at(1).toJSON()))

Here is the output :

{"name":"classic","tracks":[{"title":"fur elise"},{"title":"fuya"}]}
{"name":"c2c","tracks":[{"title":"fur elise"},{"title":"fuya"}]}

The problem is, as we can see on the output, the 2 playlists have the 2 tracks "fur elise" and "fuya".

So my question is why ? and what should I do in order to have "fur elise" only in the first playlist named "classic" and "fuya" only in the second playlist named "c2c" ?

Thank you.

I think your problem is your default tracks attribute in PlayList:

var Playlist = Backbone.Model.extend({
    defaults : {
        name : "",
        tracks : new TrackCollection,
    }
});

Backbone will shallow-copy the defaults when creating new instances:

defaults model.defaults or model.defaults()
[...]
Remember that in JavaScript, objects are passed by reference, so if you include an object as a default value, it will be shared among all instances.

The result is that every single PlayList instance that uses the default tracks will be using exactly the same TrackCollection as its tracks attribute and that TrackCollection will be the one referenced in the defaults.

The easiest solution is to use a function for defaults:

var Playlist = Backbone.Model.extend({
    defaults : function() {
        return {
            name : "",
            tracks : new TrackCollection,
        };
    }
});

That way the defaults function will be called when defaults are needed and each time defaults is called you'll get a brand new TrackCollection in the default tracks attribute.

Here's a quick rule of thumb for you: