message.rb

  scope :unread_for_user, lambda { |user|
    if user
      {
        :joins => "left outer join read_receipts rr on messages.id = rr.readable_id and rr.readable_type = 'message' and rr.user_id = #{user.id}",
        :conditions => "rr.id is NULL"
      }
    end
  }