Add controller for cart and beginnings of an API.

app/controllers/cart_controller.rb

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+class CartController < Spree::Api::BaseController
+  respond_to :json
+
+  def new
+    @cart = Cart.new(current_api_user)
+    if @cart.save
+      respond_with(@cart, :status => 201)
+    else
+      invalid_resource!(@cart)
+    end
+  end
+
+  def show
+    @cart = Cart.find(params[:id])
+    respond_with(@cart)
+  end
+
+  def add_product
+  end
+
+end

app/views/cart/show.v1.rabl

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+object @cart
+attributes :id
+
+node( :orders ) do |p|
+  partial '/orders/index', object: p.orders
+end

app/views/orders/index.v1.rabl

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+collection @orders
+
+extends "orders/show"

app/views/orders/show.v1.rabl

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+object @order
+attributes :id
+
+node( :distributor ) { |p| p.distributor.blank? ? "" : p.distributor.name }

spec/controllers/cart_controller_spec.rb

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+require 'spec_helper'
+require 'spree/api/testing_support/helpers'
+
+describe CartController do
+  include Spree::Api::TestingSupport::Helpers
+  render_views
+
+  let(:current_api_user) { stub_model(Spree.user_class, :email => "spree@example.com") }
+  let!(:product1) { FactoryGirl.create(:product) }
+  let!(:cart) { Cart.create(user: current_api_user) }
+
+  before do
+    stub_authentication!
+    Spree.user_class.stub :find_by_spree_api_key => current_api_user
+  end
+
+  context "as a normal user" do
+
+
+    context 'with an existing cart' do
+
+      it "retrieves an empty cart" do
+        spree_get :show, {id: cart, :format => :json }
+
+        json_response["id"].should == cart.id
+        json_response['orders'].size.should == 0
+      end
+
+      context 'with an order' do
+
+        let(:order) { FactoryGirl.create(:order_with_totals_and_distributor) }
+
+        before(:each) do
+          cart.orders << order
+          cart.save!
+        end
+
+        it "retrieves a cart with a single order and line item" do
+          spree_get :show, {id: cart, :format => :json }
+
+          json_response['orders'].size.should == 1
+          json_response['orders'].first['distributor'].should == order.distributor.name
+        end
+      end
+    end
+  end
+end