问请求规范中的存根身份验证

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require File.dirname(__FILE__) + '/../spec_helper'
require File.dirname(__FILE__) + '/authentication_helpers'


describe "Messages" do
  include AuthenticationHelpers

  describe "GET admin/messages" do
    before(:each) do
      @current_user = Factory :super_admin
      login(@current_user)
    end

    it "displays received messages" do
      sender = Factory :jonas
      direct_message = Message.new(:sender_id => sender.id, :subject => "Message system.", :content => "content", :receiver_ids => [@current_user.id])
      direct_message.save
      get admin_messages_path
      response.body.should include(direct_message.subject) 
    end
  end
end

module AuthenticationHelpers
  def login(user)
    session[:user_id] = user.id # session is nil
    #controller.stub!(:current_user).and_return(user) # controller is nil
  end
end

class ApplicationController < ActionController::Base
  protect_from_forgery

  helper_method :current_user
  helper_method :logged_in?

  protected

  def current_user  
    @current_user ||= User.find(session[:user_id]) if session[:user_id]  
  end

  def logged_in?
    !current_user.nil?
  end
end

1) Messages GET admin/messages displays received messages
     Failure/Error: login(@current_user)
     NoMethodError:
       undefined method `session' for nil:NilClass
     # ./spec/requests/authentication_helpers.rb:3:in `login'
     # ./spec/requests/message_spec.rb:15:in `block (3 levels) in <top (required)>'