The not so famous before_remove/before_add & after_remove/after_add callbacks.

Well, this is another rails feature and a quite nice one which is not documented properly.

While using has_many :through associations, we can use before_remove, before_add, after_remove, after_add callbacks.


We can pass methods to these callbacks which would be called when a new association is added or removed.

The fun part here is, the after/before_remove callback works even when #delete method is called on an object. How about that!!

So, if you have an association like:

..............................................................................................................................................

class Foo < ActiveRecord::Base

.......

has_many :bars, :through => foo_bars, :after_remove => :remove_deleted_foo_bars

-----

def remove_deleted_foo_bars(record)   <---- This parameter has to be passed
   //perform action
end
...................................................................................................................................................

Also, should any of the before_add callbacks throw an exception, the object does not get added to the collection. Same with the before_remove callbacks; if an exception is thrown the object doesn’t get removed. <---- This is from the rails docs ;)

In my case, i had to expire cache when the association was deleted, so this came in really handy to me. Hope this helps you all :)