application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery
  layout Proc.new { |controller| controller.request.xhr? ? 'ajax' : 'test' }
end

specs

Failures:

  1) StoresController GET render_implicit_html_template_from_xhr_request with XHR should render the ajax layout
     Failure/Error: response.should render_template("layouts/ajax")
       Expected block to return true value.
     # ./spec/controllers/stores_controller_spec.rb:29:in `block (3 levels) in <top (required)>'

Finished in 0.35621 seconds
3 examples, 1 failure

stores_controller.rb

class StoresController < ApplicationController
  # Layout inheritance is not working.
  # Please see https://github.com/rails/rails/issues/1685
  # layout Proc.new { |controller| controller.request.xhr? ? 'ajax' : 'application' }

  def render_implicit_html_template_from_xhr_request
  end
end

stores_controller_spec.rb

require "spec_helper"

describe StoresController do
  include Devise::TestHelpers
  render_views

  before (:each) do
    @user = Factory.create(:user)
    sign_in @user
  end

  context "GET render_implicit_html_template_from_xhr_request with XHR" do
    before(:each) do
      xhr :get, :render_implicit_html_template_from_xhr_request
    end
    it {should respond_with :success }
    it {should render_template(:render_implicit_html_template_from_xhr_request) }
    it "should render the ajax layout" do
      response.should render_template("layouts/ajax") # fails
      # response.should render_template("layouts/test") # passes
    end
  end
end