Cascade delete in Ruby ActiveRecord models?

Question

I was following the screencast on rubyonrails.org (creating the blog).

I have following models:

comment.rb

class Comment < ActiveRecord::Base     belongs_to :post     validates_presence_of :body # I added this end 

post.rb

class Post < ActiveRecord::Base     validates_presence_of :body, :title     has_many :comments end 

Relations between models work fine, except for one thing - when I delete a post record, I'd expect RoR to delete all related comment records. I understand that ActiveRecords is database independent, so there's no built-in way to create foreign key, relations, ON DELETE, ON UPDATE statements. So, is there any way to accomplish this (maybe RoR itself could take care of deleting related comments? )?

Answer 1

Yes. On a Rails' model association you can specify the :dependent option, which can take one of the following three forms:

• :destroy/:destroy_all The associated objects are destroyed alongside this object by calling their destroy method
• :delete/:delete_all All associated objects are destroyed immediately without calling their :destroy method
• :nullify All associated objects' foreign keys are set to NULL without calling their save callbacks

Note that the :dependent option is ignored if you have a :has_many X, :through => Y association set up.

So for your example you might choose to have a post delete all its associated comments when the post itself is deleted, without calling each comment's destroy method. That would look like this:

class Post < ActiveRecord::Base   validates_presence_of :body, :title   has_many :comments, :dependent => :delete_all end 

Update for Rails 4:

In Rails 4, you should use :destroy instead of :destroy_all.

If you use :destroy_all, you'll get the exception:

The :dependent option must be one of [:destroy, :delete_all, :nullify, :restrict_with_error, :restrict_with_exception]