api_controller_test.rb

module APIControllerTest
  def get_json(action, params = {})
    get action, params.merge(:format => :json)
    assert_response :success
    JSON.parse(@response.body)
  end

  def assert_status(expected_code)
    assert_equal expected_code, @response.status
  end
end

items_controller_test.rb

require 'test_helper'

class ItemsControllerTest < ActionController::TestCase
  include APIControllerTest
  
  def test_it_lists_items
    get_json(:index)
    assert_status 200
  end
end

orders_controller_test.rb

require 'test_helper'

class OrdersControllerTest < ActionController::TestCase
  include APIControllerTest
  
  def test_it_shows_orders
    2.times{ Order.create }
    data = get_json(:index)
    assert_equal 2, data.count
  end

  def test_it_responds_with_a_single_order
    order = Order.create
    data = get_json(:show, :id => order.id)
    assert_equal order.id, data["id"]
  end

  def test_it_creates_an_order
    params = {:order => {:amount => 9.00, :user_id => 2}, :format => :json}
    post :create, params
    assert_status 201
  end
end