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,
})

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()))

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

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

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