var Model = Backbone.Model.extend({
collection: null,
initialize: function(){
this.collection = new Collection();
}
});
var Collection = Backbone.Collection.extend({
model: Model
});
var collection = new Collection([{}]),
submodel = new Model({});
collection.add(submodel);
collection.at(0).collection.add(submodel);
console.log(collection.at(1) === collection.at(0).collection.at(0)); // true