来自self.id的未定义方法`id'

Question

我正在从头开始构建一个收件箱消息传递系统，当我将self.id添加到模型中的条件时，我在MessagesController中标题为NoMethodError的所有页面上都会出现一个未定义的方法`id'错误。虽然我无法控制它，但我尝试了一些解决方法。

messages_controller：

class MessagesController < ApplicationController

  before_filter :set_user

  def index
    if params[:mailbox] == "sent"
      @messages = @user.sent_messages
    elsif params[:mailbox] == "inbox"
      @messages = @user.received_messages
    #elsif params[:mailbox] == "archieved"
     # @messages = @user.archived_messages
    end
  end

  def new
    @message = Message.new
    if params[:reply_to]
      @reply_to = User.find_by_user_id(params[:reply_to])
      unless @reply_to.nil?
        @message.recepient_id = @reply_to.user_id
      end
    end
  end

  def create
    @message = Message.new(params[:message])
    @message.sender_id = params[:user_id]
    if @message.save
      flash[:notice] = "Message has been sent"
      redirect_to user_messages_path(current_user, :mailbox=>:inbox)
    else
      render :action => :new
    end
  end

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

  def delete_multiple
      if params[:delete]
        params[:delete].each { |id|
          @message = Message.find(id)
          @message.mark_message_deleted(@message.id,@user_id) unless @message.nil?
        }
        flash[:notice] = "Messages deleted"
      end
      redirect_to user_messages_path(@user, @messages)
  end

  private
    def set_user
      @user = current_user
    end
end

user.rb：

:conditions => ["messages.recipient_deleted = ? AND messages.recipient_id = ?",false,self.id]
..
 ..

 def unread_messages?
   unread_message_count > 0 ? true : false
 end

 # Returns the number of unread messages for this user
 def unread_message_count
   eval 'messages.count(:conditions => ["recepient_id = ? AND read_at IS NULL", self.user_id])'
 end