ruby-on-rails - 为 has_many 或 habtm 动态创建 after_add 和 after_remove 回调？

Question

有没有办法动态添加after_add和 after_remove对现有 has_many 的回调或 has_and_belongs_to_many关系？

例如，假设我有模型 User , Thing , 和一个连接模型 UserThingRelationship ，以及 User模型是这样的:

class User < ActiveRecord::Base
  has_many :user_thing_relationships
  has_many :things, :through => :user_thing_relationships
end

我希望能够在一个扩展 User 的模块中, 添加 :after_add和 :after_remove回调到 User.has_many(:things, ...)关系。即，有类似的东西

module DoesAwesomeStuff
  def does_awesome_stuff relationship, callback
    # or however this can be achieved...
    after_add(relationship) callback
    after_remove(relationship) callback
  end
end

以便

class User < ActiveRecord::Base
  has_many :user_thing_relationships
  has_many :things, :through => :user_thing_relationships

  does_awesome_stuff :things, :my_callback
  def my_callback; puts "awesome"; end
end

实际上与

class User < ActiveRecord::Base
  has_many :user_thing_relationships
  has_many :things, :through => :user_thing_relationships, :after_add => :my_callback, :after_remove => :my_callback

  def my_callback; puts "awesome"; end
end

这可以非常有效地添加 after_save等，对正在扩展的模型的回调，因为 ActiveRecord::Base#after_save只是一个类方法。

Answer 1

最简单的是

User.after_add_for_things << lambda do |user, thing| 
  Rails.logger.info "#{thing} added to #{user}"
end

注意:不再适用于 Rails 7(感谢 Andrew Hodgkinson 指出这一点)