为 has_many 或 habtm 动态创建 after_add 和 after_remove 回调？

Question

有没有办法将after_add 和after_remove 回调动态添加到现有的has_many 或has_and_belongs_to_many 关系中？

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

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

我希望能够在扩展User 的模块中，将:after_add 和:after_remove 回调添加到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

这样

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

相同

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

这可以非常有效地为正在扩展的模型添加after_save 等回调，因为ActiveRecord::Base#after_save 只是一个类方法。

Answer 1

最简单的应该是

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

Answer 2

我能够通过使用ActiveRecord::Reflection 提出以下建议：

module AfterAdd
  def after_add rel, callback
    a = reflect_on_association(rel)
    send(a.macro, rel, a.options.merge(:after_add => callback))
  end
end

class User < ActiveRecord::Base
  extend AfterAdd

  has_many :user_thing_relationships
  has_many :things, :through => :user_thing_relationships

  after_add :things, :my_callback

  def my_callback
    puts "Hello"
  end
end

我不想回答我自己的问题，所以如果其他人在接下来的几天内能提出更好的解决方案，我不会给自己的答案。