这两个模块是用来处理用户登录的信息存储，代码是参考 rubyonrailstoturial 第 8 章的内容。不知道为什么当程序运行到 SessionHelper 中的 sign_in 方法的时候， user.update_attributes(...)这句总是提示 NameError (uninitializedconstant SessionsHelper::Users)错误，求教！

class SessionsController < ApplicationController
   def new                                  
   end                                                                   

  def create                                                            
   user = User.find_by(email: params[:session][:email].downcase)       
    if user && user.authenticate(params[:session][:password])           
     sign_in  user                                                      
    redirect_to user                                                  
   else                        
    flash.now[:error] = 'Invalid email/password combination' #Not quite right!
     render 'new'                                                       
    end 
  end                                                                   

  def destroy                                                           
  end 

 end   

module SessionsHelper
     def sign_in(user)
      remember_token = User.new_remember_token
      cookies.permanent[:remember_token] = remember_token
      self.current_user = user
    user.update_attribute(:remember_token, Users.encrypt(remember_token)) 
      end

    def signed_in?
     !current_user.nil?
    end

    def current_user=(user)
      @current_user = user
    end

    def current_user
      remember_token = User.encrypt(cookies[:remember_token])
      @current_user ||= User.find_by(remember_token: remember_token)
    end
  end