Rails habtm callbacks

Question

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 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?

Capello

Answer 1

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

Bide

does it matter on which side of the relation do I add the callback, or is it the same?? – Calistacalisthenics

I just tried this with Rails 3.2.8 and it sadly mattered, on which side you add those callbacks. What is your experience? – Shape

For anyone stumbling on this now, yes it matters. That's the trap I fell into. See this answer to understand #56952426 – Recur

For the ones that were stuck with the wrong number of arguments (given 1, expected 0) error, need to include the arg in the private methods called from the callbacks, as it's specified here api.rubyonrails.org/classes/ActiveRecord/Associations/… – Madelaine

the record that is being added or removed, can be passed as a parameter to the callback function. In the above case, tweet_promotion can be passed the role record when role is added/removed for a user record in HABTM, function signature will look like tweet_promotion(role). The same can be done for the opposite direction. – Polonium