实现简单的私人消息后，未定义的方法`error_message_on'

Question

我正在开发我的第一个RoR网站，然后我完成了安装simple_private_messages的步骤。这是它看起来的样子吗？ http://i.imgur.com/HTootKv.jpg

点击“撰写”时收到此错误消息：

 NoMethodError in Messages#new

Showing C:/app/views/messages/new.html.erb where line #5 raised:

undefined method `error_message_on' for #<#<Class:0x516e6d0>:0x516c0f0>

我正确地完成了所有步骤（我相信）。

这是我的route.db

  resources :users do
             resources :messages do
               collection do
                 post :delete_selected
               end
             end
           end 

消息/ new.html.erb

<% form_for @message, :url => user_messages_path(@user) do |f| %>
  <p>
    To:<br />
        <%= f.text_field :to %>
        <%= error_message_on @message, :to %>
  </p>
  <p>
    Subject:<br />
    <%= f.text_field :subject %>
    <%= error_message_on @message, :subject %>
    </p>
    <p>
      Message<br />
      <%= f.text_area :body %>
            <%= error_message_on @message, :body %>
    </p>
    <p>
      <%= submit_tag "Send" %>
    </p>
<% end %>

消息模型

class Message < ActiveRecord::Base

  is_private_message

  attr_accessor :to

end

消息控制器

class MessagesController < ApplicationController

  before_filter :set_user

  def index
    if params[:mailbox] == "sent"
      @messages = @user.sent_messages
    else
      @messages = @user.received_messages
    end
  end

  def show
    @message = Message.read_message(params[:id], current_user)
  end

  def new
    @message = Message.new

    if params[:reply_to]
      @reply_to = @user.received_messages.find(params[:reply_to])
      unless @reply_to.nil?
        @message.to = @reply_to.sender.login
        @message.subject = "Re: #{@reply_to.subject}"
        @message.body = "\n\n*Original message*\n\n #{@reply_to.body}"
      end
    end
  end

  def create
    @message = Message.new(params[:message])
    @message.sender = @user
    @message.recipient = User.find_by_login(params[:message][:to])

    if @message.save
      flash[:notice] = "Message sent"
      redirect_to user_messages_path(@user)
    else
      render :action => :new
    end
  end

  def delete_selected
    if request.post?
      if params[:delete]
        params[:delete].each { |id|
          @message = Message.find(:first, :conditions => ["messages.id = ? AND (sender_id = ? OR recipient_id = ?)", id, @user, @user])
          @message.mark_deleted(@user) unless @message.nil?
        }
        flash[:notice] = "Messages deleted"
      end
      redirect_to :back
    end
  end

  private
    def set_user
      @user = User.find(params[:user_id])
    end
end