Backbone.js Collection of Collections

I'm trying to figure out how to make a Collection of collections with backbone.js. I'm pretty new to backbone. I have something like the following situation:

 +---------------+               +------------------+
 | Playlists     |               | Playlist         |
 |---------------|          0..* |------------------|
 |               +-------------->| Name             |
 |               |               |                  |
 |               |               |                  |
 +---------------+               +-------+----------+
                                         |
                                         |
                                         |0..*
                                         v
                                 +------------------+
                                 |  Track           |
                                 |------------------|
                                 | Name             |
                                 | Artist           |
                                 |                  |
                                 +------------------+

In code this looks similar to this:

var trackModel = Backbone.Model.extend({
    //trackdata
});

var playlistModel = Backbone.Collection.extend({
    model : trackModel,
    url   : "playlist"
});

var playlistsModel = Backbone.Collection.extend({
    url   : "playlists",
    model : playlistModel   //This pretty sure doesn't work like I want, because there is no model attribute for collections :S
});

However I always receive an error in the js console saying:

 Uncaught TypeError: Object [object Object] has no method '_validate'

when I try to execute a function that triggers the validate (like add, fetch, ...)

It makes no difference if i add the validate or _validate function to any of the collections or models.

I believe this is because backbone.js doesn't support collections in collections. Is there another way that works?

UPDATE:

This is how it looks right now

var Track = Backbone.Model.extend({ 
    //trackdata 
}); 

var Tracks = Backbone.Collection.extend({ 
    model:Track; 
}); 

var Playlist = Backbone.Model.extend({ 
    //name  : ...
    tracks: new Tracks ()
}); 

var Playlists = Backbone.Collection.extend({ 
    url : "playlists", 
    model : Playlist 
});

You'd solve your problem by turning your Playlist from a collection into a model. If you think about it, a Playlist would probably have other attributes anyway (e.g. name) that wouldn't be settable on a collection.

Playlists would then be a collection of Playlist models (instead of collections), which should work without error.

var Track = Backbone.Model.extend({
    //trackdata
});

var Playlist = Backbone.Model.extend({
    model : Track
});

var Playlists = Backbone.Collection.extend({
    url   : "playlists",
    model : Playlist
});