雑なRSpec( http://qiita.com/jnchito/items/a90b3b09d008227d3d60 )の一例です

api_products_spec.rb

require 'rails_helper'

describe 'Api::Products' do
  let(:product) {create :product, name: 'AB-123CD'}
  let!(:detail) {product.create_detail(style: '屋外用')}
  let(:sub_product) {create :sub_product, name: 'AB-456UV'}
  let!(:sub_detail) {sub_product.create_sub_detail(style: '据置型')}

  describe 'index' do
    example do
      get products_path(q: 'ab', role: 'admin')
      expect(response).to have_http_status :ok
      result = JSON.parse(response.body)
      data = Hashie::Mash.new(result)
      expect(data.products.size).to eq 2

      product_1 = data.products[0]
      expect(product_1.type).to eq 'Product'
      expect(product_1.name).to eq 'AB-123CD'
      expect(product_1.style).to eq '屋外用'

      product_2 = data.products[1]
      expect(product_2.type).to eq 'SubProduct'
      expect(product_2.name).to eq 'AB-456UV'
      expect(product_2.style).to eq '据置型'

      # detailがないとき
      detail.destroy
      sub_detail.destroy

      get products_path(q: 'ab', role: 'admin')
      expect(response).to have_http_status :ok
      result = JSON.parse(response.body)
      data = Hashie::Mash.new(result)
      expect(data.products.size).to eq 2

      product_1 = data.products[0]
      expect(product_1.type).to eq 'Product'
      expect(product_1.name).to eq 'AB-123CD'
      expect(product_1.style).to be_nil

      product_2 = data.products[1]
      expect(product_2.type).to eq 'SubProduct'
      expect(product_2.name).to eq 'AB-456UV'
      expect(product_2.style).to be_nil
    end
  end

  describe 'show' do
    example do
      get product_path(product, type: 'Product', role: 'admin')
      expect(response).to have_http_status :ok
      result = JSON.parse(response.body)
      product_1 = Hashie::Mash.new(result).product

      expect(product_1.type).to eq 'Product'
      expect(product_1.name).to eq 'AB-123CD'
      expect(product_1.style).to eq '屋外用'

      get product_path(sub_product, type: 'SubProduct', role: 'admin')
      expect(response).to have_http_status :ok
      result = JSON.parse(response.body)
      product_2 = Hashie::Mash.new(result).product

      expect(product_2.type).to eq 'SubProduct'
      expect(product_2.name).to eq 'AB-456UV'
      expect(product_2.style).to eq '据置型'

      # detailがないとき
      detail.destroy
      sub_detail.destroy

      get product_path(product, type: 'Product', role: 'admin')
      expect(response).to have_http_status :ok
      result = JSON.parse(response.body)
      product_1 = Hashie::Mash.new(result).product

      expect(product_1.type).to eq 'Product'
      expect(product_1.name).to eq 'AB-123CD'
      expect(product_1.style).to be_nil

      get product_path(sub_product, type: 'SubProduct', role: 'admin')
      expect(response).to have_http_status :ok
      result = JSON.parse(response.body)
      product_2 = Hashie::Mash.new(result).product

      expect(product_2.type).to eq 'SubProduct'
      expect(product_2.name).to eq 'AB-456UV'
      expect(product_2.style).to be_nil
    end
  end

  describe 'エラー処理' do
    context 'レコードが見つからない場合' do
      example do
        get product_path(-1, type: 'Product', role: 'admin')
        expect(response).to have_http_status :not_found
        result = JSON.parse(response.body)
        expect(result['message']).to eq 'Not Found'
      end
    end
    context 'typeの指定がおかしい場合' do
      example do
        get product_path(product, type: 'FooBar', role: 'admin')
        expect(response).to have_http_status :not_found
        result = JSON.parse(response.body)
        expect(result['message']).to eq 'Not Found'
      end
    end
    context '権限がない場合' do
      example do
        get product_path(product, type: 'Product')
        expect(response).to have_http_status :forbidden
        result = JSON.parse(response.body)
        expect(result['message']).to eq 'Forbidden'
      end
    end
    context '実行時エラーが発生した場合' do
      example do
        allow(UserContext).to receive(:new).and_raise('TEST.')
        get product_path(product, type: 'Product', role: 'admin')
        expect(response).to have_http_status :internal_server_error
        result = JSON.parse(response.body)
        expect(result['message']).to eq 'TEST.'
      end
    end
  end
end