基于关联的ActiveRecord计算字段

我有一个会话ActiveRecord模型，基本上如下

class Conversation < ActiveRecord::Base
  has_many :messages, :order => 'created_at ASC'
  belongs_to :latest_message, :class_name => 'Message'
end

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有什么想法吗？

在我写问题时找到了答案：

  has_many :messages, :order => 'created_at ASC',
           :after_add => :update_latest_message, :after_remove => :update_latest_message

  def update_latest_message(message)
    self.latest_message = self.messages.last
    self.save
  end

ActiveRecord关联回调允许我调用一个在必要时更新消息的方法。如果我不用再打电话给save就好了

  has_many :messages, :order => 'created_at ASC',
           :after_add => :update_latest_message, :after_remove => :update_latest_message

  def update_latest_message(message)
    self.latest_message = self.messages.last
    self.save
  end