如何使用Rails查找一个has_many而不是另一个？

class User < ActiveRecord::Base

    has_many :sent_messages, :class_name => \"Message\", :foreign_key => \"sender_id\"
    has_many :receivers, :through => :sent_messages, :source => :receiver do 
        def for_message(message_id)
            where(\"messages.id = ?\", message_id)
        end
    end

    # Another association mapping for received_messages

end


class Message < ActiveRecord::Base

    belongs_to :sender, :class_name => \"User\", :foreign_key => \"sender_id\"
    belongs_to :receiver, :class_name => \"User\", :foreign_key => \"receiver_id\"

end


#Usage
User.first.sent_messages

#all users received messages from first user
User.first.receivers

#all users received messages from first user for message of message_id
User.first.receivers.for_message(message_id)

def other_user(one_user)
  user_messages.reject do |msg|
    msg.user_id == one_user.id
  end.first.user
end

user_messages

one_user

Message

sender_id

receiver_id

UserMessage

class User
  has_many :sent_messages, :class_name => \'Message\', :foreign_key => \'sender_id\'
  has_many :received_messages, :class_name => \'Message\', :foreign_key => \'receiver_id\'
end

class Message
  belongs_to :sender, :class_name => \'User\'
  belongs_to :recipient, :class_name => \'User\'
end

SentMessageLink

has_many :through

class SentMessageLink < UserMessage
  belongs_to :sender, :class_name => \'User\'
  belongs_to :message
end

class User
  has_many :sent_message_links
  has_many :sent_messages, :through => :sent_message_links
end

class Message
  has_one :sent_message_link
  has_one :sender, :through => :sent_message_link
end

sender

receiver