Mixins blogpost

digitalAssetsCollection.js

var DigitalAssetsCollection = Backbone.Collection.extend({
  model: Backbone.Model,
  initialize: function(undefined, options) {
    this.parent = options && options.parent;
    if ( !this.parent ) {
      throw "a parent product must be provided at initialization";
    }

    _.bindAll(this, "url");
  },

  // Uses the path of the parent to generate the url for this digital
  // assets collection
  url: function() {
    return _.result(this.parent, "url") + "/digital_assets";
  }
});

mixinDigitalAssetsCollection.js

var DigitalAssetsCollection = Backbone.Collection.extend({
  model: Backbone.Model,
  mixins: [ NestedModelMixin ]
});

nestedModelMixin.js

var NestedModelMixin = {
  initialize: function(undefined, options) {
    this.parent = options && options.parent;
    if ( !this.parent || ! this.nestedPath ) {
      throw "a parent model, corresponding to the parent of this Nested, " +
        "must be provided at initialization, as well as the nestedPath";
    }

    _.bindAll(this, "url");
  },

  // Path to query against is the root path of the parent model, plus the collection key, e.g.
  // [/products/999] + [/digital_assets] would be /products/999/digital_assets; query parameters
  // returned by `params` below would then be tacked on, if returned.
  url: function() {
    return _.result(this.parent, "url") + "/" + this.nestedPath;
  }

};