api_constraints.rb

class ApiConstraints
  def initialize(options)
    @version = options[:version]
    @default = options[:default]
  end

  def matches?(req)
    @default || req.headers['Accept'].include?("application/vnd.marketplace.v#{@version}")
  end
end

routes.rb

require 'api_constraints'

MarketPlaceApi::Application.routes.draw do
  devise_for :users
  # Api definition
  namespace :api, defaults: { format: :json },
                              constraints: { subdomain: 'api' }, path: '/'  do
    scope module: :v1,
              constraints: ApiConstraints.new(version: 1, default: true) do
      # We are going to list our resources here
      resources :users, :only => [:show, :create]
    end
  end
end

spec_helper.rb

# This file is copied to spec/ when you run 'rails generate rspec:install'
ENV["RAILS_ENV"] ||= 'test'
require File.expand_path("../../config/environment", __FILE__)
require 'rspec/rails'

# Requires supporting ruby files with custom matchers and macros, etc, in
# spec/support/ and its subdirectories. Files matching `spec/**/*_spec.rb` are
# run as spec files by default. This means that files in spec/support that end
# in _spec.rb will both be required and run as specs, causing the specs to be
# run twice. It is recommended that you do not name files matching this glob to
# end with _spec.rb. You can configure this pattern with with the --pattern
# option on the command line or in ~/.rspec, .rspec or `.rspec-local`.
Dir[Rails.root.join("spec/support/**/*.rb")].each { |f| require f }

# Checks for pending migrations before tests are run.
# If you are not using ActiveRecord, you can remove this line.
ActiveRecord::Migration.maintain_test_schema!

RSpec.configure do |config|
  # ## Mock Framework
  #
  # If you prefer to use mocha, flexmock or RR, uncomment the appropriate line:
  #
  # config.mock_with :mocha
  # config.mock_with :flexmock
  # config.mock_with :rr

  # Remove this line if you're not using ActiveRecord or ActiveRecord fixtures
  config.fixture_path = "#{::Rails.root}/spec/fixtures"

  # If you're not using ActiveRecord, or you'd prefer not to run each of your
  # examples within a transaction, remove the following line or assign false
  # instead of true.
  config.use_transactional_fixtures = true

  config.include Devise::Test::ControllerHelpers, type: :controller

  # If true, the base class of anonymous controllers will be inferred
  # automatically. This will be the default behavior in future versions of
  # rspec-rails.
  config.infer_base_class_for_anonymous_controllers = false

  # Run specs in random order to surface order dependencies. If you find an
  # order dependency and want to debug it, you can fix the order by providing
  # the seed, which is printed after each run.
  #     --seed 1234
  config.order = "random"

  # RSpec Rails can automatically mix in different behaviours to your tests
  # based on their file location, for example enabling you to call `get` and
  # `post` in specs under `spec/controllers`.
  #
  # You can disable this behaviour by removing the line below, and instead
  # explictly tag your specs with their type, e.g.:
  #
  #     describe UsersController, :type => :controller do
  #       # ...
  #     end
  #
  # The different available types are documented in the features, such as in
  # https://relishapp.com/rspec/rspec-rails/v/3-0/docs
  config.infer_spec_type_from_file_location!
end

users_controller.rb

class Api::V1::UsersController < ApplicationController
  respond_to :json

  def show
    respond_with User.find(params[:id])
  end

  def create
    user = User.new(user_params)
    if user.save
      render json: user, status: 201, location: [:api, user]
    else
      render json: { errors: user.errors }, status: 422
    end
  end

  private

  def user_params
    params.require(:user).permit(:email, :password, :password_confirmation)
  end
end

users_controller_spec.rb

require 'spec_helper'

describe Api::V1::UsersController do

  before(:each) { request.headers['Accept'] = "application/vnd.marketplace.v1" }

  describe "GET #show" do
    before(:each) do
        @user = FactoryGirl.create :user
        get :show, id: @user.id, format: :json
      end

    it "returns the information about a reporter on a hash" do
        user_response = JSON.parse(response.body, symbolize_names: true)
        expect(user_response[:email]).to eql @user.email
      end

    it { should respond_with 200 }
  end

  describe "POST #create" do

    context "when is successfully created" do
      before(:each) do
        @user_attributes = FactoryGirl.attributes_for :user
        post :create, { user: @user_attributes }, format: :json
      end

      it "renders the json representation for the user record just created" do
        user_response = JSON.parse(response.body, symbolize_names: true)
        expect(user_response[:email]).to eql @user_attributes[:email]
      end

      it { should respond_with 201 }
    end

    context "when is not created" do
      before(:each) do
        #notice I'm not including the email
        @invalid_user_attributes = { password: "12345678",
                                     password_confirmation: "12345678" }
        post :create, { user: @invalid_user_attributes }, format: :json
      end

      it "renders an errors json" do
        user_response = JSON.parse(response.body, symbolize_names: true)
        expect(user_response).to have_key(:errors)
      end

      it "renders the json errors on why the user could not be created" do
        user_response = JSON.parse(response.body, symbolize_names: true)
        expect(user_response[:errors][:email]).to include "can't be blank"
      end

      it { should respond_with 422 }
    end
  end
end