[PATCH] belongs_to association supports before_add and after_add callbacks

code and tests for before_add and after_add callbacks on belongs_to associations

I'd love to have the :before_remove and :after_remove callbacks. Any chance you could make those happen, or do you need more demand than just me? :-)

A question: In your example, would:

wikipage.comments << comment2

or

wikipage.comments.create()

...trigger the belongs_to :after_add callback, or not?

Thanks.

Andrew

Andrew, I'm not sure... I'm guessing yes, but it's been a while since I've used this code, so things may have changed. It should be trivial to add a test case that exercises the other side of the association. If I see interest in this patch picking up I'll be motivated to add the other callbacks, although this might be a good place for someone new to Rails' internals to dive in.

The work to add before and after remove callbacks would mirror the before and after add callbacks, so you can probably figure it out by reading this patch. Could be a good afternoon project.

I guess I could've tried before I asked... looks like the belongs_to callbacks aren't triggered when the has_many object's << or .delete methods are called. Too bad. That means I'm left still looking for a callback-y thingie that's triggered whenever an association is changed, from whatever direction. (Or, rather, I'm stuck writing a motley assortment of callbacks in different classes to cover every possibility.)

I only started with Ruby and Rails about a week ago, so I'm not sure I'm quite ready to dig into the internals yet. Too many dragons. ;-)

I also think this would be useful. I have a join object with a polymorphic end on one side. I would like to be able to use after_add on the belongs_to but it does look like it exists. Since it's polymorphic, I can't really add a after_add on the other end.

There any plans to add this sooner than 2.x? I'm needing to trigger a model field change based on the changing of the reference to target of a belongs_to association and I've been banging my head against the wall trying to do this. I can't help thinking that having before/after add/remove callbacks would make this a lot easier. Though I have to admit this is probably a fairly rare use case.

+1 on this if it helps.

Looks nice. Doesn't apply cleanly to current edge, though. Also, I think you should either implement or entirely remove the commented out *_remove bits.

If you update this to apply cleanly, I'll apply it, run the test suite and will +1 if it passes.

+1

just got a huge headache because this is not in yet (and can't use a custom rails version)

pedrobelo, you might be able to code round this, depending on how you need to use it, by aliasing the association writer method.

Something like this, the changing of Model#association should reset the value of Model#some_other_field

def association_with_trigger=(val)
  returning(self.send(:association_without_trigger=, val)) do
    if @association && @association.updated?
      @flag_some_other_field_should_change = true
    end
  end
end
alias_method_chain :association=, :trigger

before_save :act_on_flags

def act_on_flags
  if @flag_some_other_field_should_change
    @flag_some_other_field_should_change = false
    self.some_other_field = 'cheese'
  end
end

tweak as required

got it, thanks.

(but still +1! heh)

http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/1564-add-support-for-before-and-after-addremove-callbacks-to-belongs_to-associations