Tell me how to use ActiveRecord#afeter_add

Now I hava a problem,how can I make the callback#after_add receive a reference to the join model in a has_many :through association? my code like this:

   class Emergency
     has_many :departments, :through => :eme_references, :after_add => Proc.new { |eme_reference| eme_reference.eme_flag = 1}
  end

the attribute eme_flag is the model EmeReference's attribute! but in the block ,i get the eme_reference.class is Emergency. I want to set the attribute eme_flag of the model EmeReference. That is my question! cheers!

Presumably Emergency also has_many :eme_references in order for the :through association to work?

In that case, you should be able to attach the callback there:

has_many :eme_references, 
  :after_add => Proc.new { |emergency, eme_ref| # code here }

The block accepts 2 parameters, the first will be the Emergency, the 2nd will be the EmeReference being added.

Perhaps a before_save callback on EmeReference can also do what you want in this instance?