Rails habtm callbacks

Is there a way to add callbacks for when an item is added to a habtm relationship?

For example, I have the following two models, User and Role:

# user.rb
class User; has_and_belongs_to_many :roles; end

 

# role.rb
class Role; has_and_belongs_to_many :users; end

I want to add a callback to the << method (@user << @role), but I can't seem to find an ActiveRecord callback because there 开发者_开发问答is no model for the join table (because its a true habtm).

I'm aware that I could write a method like add_to_role(role), and define everything in there, but I'd prefer to use a callback. Is this possible?

---

Yes there is:

class User < AR::Base
  has_and_belongs_to_many :roles, 
    :after_add => :tweet_promotion, 
    :after_remove => :drink_self_stupid

private

  def tweet_promotion
    # ...
  end

  def drink_self_stupid
    # ...
  end
end

Look for 'Association callbacks' on this page for more: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html