Question

I want to run some code before an object is removed from a has_many association.

I thought that I would be able to do this with the before_remove callback however for some reason this isn’t firing and I don’t understand why.

class Person < ActiveRecord::Base
  has_many :limbs, before_remove: :print_message

  def print_message
    puts 'removing a limb'
  end
end

class Limb < ActiveRecord::Base
  belongs_to :person
end

While this code should print “removing a limb” during the destruction of a limb but it doesn’t.

p = Person.create;
l = Limb.create person: p;
p.limbs.first.destroy
# SQL (2.1ms)  DELETE FROM "limbs" WHERE "limbs"."id" = ?  [["id", 5]]
# => #<Limb id: 5, person_id: 3, created_at: "2012-01-17 11:28:01", updated_at: "2012-01-17 11:28:01"> 

Why does this destroy action not cause the print_message method to run?

EDIT – does this before_remove callback exist?

A number of people have asked whether this callback exists. Although I can find very few further references to it, it is documented in the Rails documentation:

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#label-Association+callbacks

It’s an association callback though rather than a root ActiveRecord callback

Edit 2 – why not just use before_destroy on Limb?

Some people are asking why I’m not using the before_destroy callback on Limb. The reason is that I want person to check that there is a minimum number of limbs and that the last one is never destroyed. This is the original problem:
How do you ensure that has_many always "has a minimum"?

Answer 1

before_remove callback exists as an option in Associations callbacks. It’s not the same as before_destroy, which is an ActiveRecord callback.

This is how you use it:

class Person < ActiveRecord::Base
  has_many :limbs, :before_remove => :print_message

  def print_message(limb)
    # limb variable references to a Limb instance you're removing
    # ( do some action here )
    # ...
  end
end

class Limb < ActiveRecord::Base
  belongs_to :person
end

You’re also calling a remove method incorrectly.

p = Person.create
l = Limb.create(:person => p)
p.limbs.first.destroy 

Here you’re calling it on Limb instance, that’s why nothing is triggered.
Call it on an association you created:

p = Person.create
l = Limb.create(:person => p)
p.limbs.destroy(l)

EDIT

For preserving minimum of associated objects you can do something like this:

class Person < ActiveRecord::Base
  has_many :limbs, :before_remove => :preserve_mimimum

  def preserve_minimum(limb)
    raise "Minimum must be preserved" if limbs.count == 1
  end
end

class Limb < ActiveRecord::Base
  belongs_to :person
end

This however does not get triggered on p.limbs.destroy_all, so you have to do something like this p.limbs.each {|l| p.limbs.destroy(l)}

Why it does not get triggered by destroy_all?

Because of this:

def destroy_all(conditions = nil)
   find(:all, :conditions => conditions).each { |object| object.destroy }
end

It iterates over each element of an association and executes destroy action on an object and not on an association, that’s why.