This blog is part of our Rails 5 series.

In Rails 4.x, it is not possible to have destroy dependency on both sides of a bi-directional association between the two models as it would result in an infinite callback loop causing SystemStackError: stack level too deep .

class User < ActiveRecord :: Base has_one :profile , dependent: :destroy end class Profile < ActiveRecord :: Base belongs_to :user , dependent: :destroy end

Calling User#destroy or Profile#destroy would lead to an infinite callback loop.

>> user = User . first => < User id: 4 , name: "George" > >> user . profile => < Profile id: 4 > >> user . destroy => DELETE FROM `profiles` WHERE `profiles` . ` id ` = 4 ROLLBACK SystemStackError: stack level too deep

Rails 5 supports bi-directional destroy dependency without triggering infinite callback loop.

>> user = User . first => < User id: 4 , name: "George" > >> user . profile => < Profile id: 4 , about: 'Rails developer' , works_at: 'ABC' > >> user . destroy => DELETE FROM "profiles" WHERE "posts" . "id" = ? [[ "id" , 4 ]] DELETE FROM "users" WHERE "users" . "id" = ? [[ "id" , 4 ]] => < User id: 4 , name: "George" >

There are many instances like above where we need to destroy an association when it is destroying itself, otherwise it may lead to orphan records.

This feature adds responsibility on developers to ensure adding destroy dependency only when it is required as it can have unintended consequences as shown below.

class User < ApplicationRecord has_many :posts , dependent: :destroy end class Post < ApplicationRecord belongs_to :user , dependent: :destroy end >> user = User . first => < User id: 4 , name: "George" > >> user . posts => < ActiveRecord :: Associations :: CollectionProxy [ < Post id: 11 , title: 'Ruby' , user_id: 4 > , #<Post id: 12, title: 'Rails', user_id: 4>]>

As we can see “user” has two posts. Now we will destroy first post.

>> user . posts . first . destroy => DELETE FROM "posts" WHERE "posts" . "id" = ? [[ "id" , 11 ]] SELECT "posts" . * FROM "posts" WHERE "posts" . "user_id" = ? [[ "user_id" , 4 ]] DELETE FROM "posts" WHERE "posts" . "id" = ? [[ "id" , 12 ]] DELETE FROM "users" WHERE "users" . "id" = ? [[ "id" , 4 ]]

As we can see, we wanted to remove post with id “11”. However post with id “12” also got deleted. Not only that but user record got deleted too.

In Rails 4.x this would have resulted in SystemStackError: stack level too deep .

So we should use this option very carefully.