/models/shopCart/Collection.js

define(
    [
        'models/shopCart/Model',
        'models/productList/productList'
    ],
    function(Model, shopListProducts) {
        return Backbone.Collection.extend({
            initialize: function(){
                var collection = this;

                collection
                    .on('add change remove', function(){
                        collection.sync();
                    })
            },
            model: Model,
            url: '/shopCart/shopCart.json',
            addProduct: function(productId){
                var product = this.get(productId);

                if (product){
                    product.set('amount', product.get('amount') + 1);
                } else {
                    product = shopListProducts.get(productId);
                    this.push(product.toJSON());
                }
            },
            removeProduct: function(productId){
                var product = this.get(productId);

                if (product.get('amount') <= 1){
                    this.remove(product);
                } else {
                    product.set('amount', product.get('amount') - 1);
                }
            },
            sync: function(){
                var collection = this;
                $.get(collection.url, collection.toJSON(), function(res){
                    if (res.success) {
                        window.localStorage.setItem('shopCartData', JSON.stringify(collection.toJSON()));
                    }
                });
            }
        });
    });