majestic.js

var Item = Backbone.Model.extend({
  defaults: {
    name: '',
    price: ''
  }
});

var ItemCollection = Backbone.Collection.extend({
  model: Item,
  url: "https://api.parse.com/1/classes/Item",
  parse: function(response){
    return response.results;
  }
});

var Order = Backbone.Model.extend({
  // define defaults as a function, otherwise the items array will be shared
  // among all orders. See http://backbonejs.org/#Model-defaults
  defaults: function(attributes){
    attributes = attributes || {};
    return _.defaults(attributes, {
      items: []
    });
  },

  addItem: function(item){
    // 1. use item.toJSON since we need to turn a model into an object that
    //    looks like {name: "Cool Food", price: 100}
    // 2. use set + concat because, if you were to just modify items in place
    //    (e.g. using .push) it wouldn't fire a change event. .concat takes an
    //    array and returns a new array of the two arrays combined, so it will
    //    fire a change event.
    this.set('items', this.get('items').concat([item.toJSON()]));
  },

  totalPrice: function(){
    return this.get('items').reduce(function(acum, item) {
      return acum + item.price;
    }, 0);
  }
});

var OrderCollection = Backbone.Model.extend({
  model: Order,
  url: "https://api.parse.com/1/classes/Order",
  parse: function(response){
    return response.results;
  }
});

var items = new ItemCollection([{name: "Cool Food", price: 10.5}]);
var order = new Order();
order.addItem(items.at(0))
console.log(order.totalPrice());