Tell me how to use ActiveRecord#afeter_add

Question

Hey,Guys! 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!

Answer 1

I think what you want to do can't be done there.

You could create an after_create hook on departments (I'm assuming Emergency has_many eme_references has_many departments):

class Emergency
  has_many :departments, :through => :eme_references
  def flag!
    eme_flag=1
    save
  end
end

class Department
  after_create :check_emergency
  # this allows you to call department.emergency. Will return nil if anything is nil
  delegate :emergency, :to =>:eme_reference, :allow_nil => true

  def check_emergency
    self.emergency.flag! if self.emergency.present?
  end
end

Answer 2

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?