habtm relationship does not support :dependent option

Question

Is it true that HABTM relationships do not support the :dependent option?

class Person < ActiveRecord::Base
  has_and_belongs_to_many :posts, :dependent => :destroy
end

I am trying rails edge.

Brainsick

As an exercise, I recommend reading this API part titled Deleting from associations, specially in the part What gets deleted?. That made me understand why and what's happening behind api.rubyonrails.org/classes/ActiveRecord/Associations/… – Floozy

Answer 1

Yep, It doesn't support it. See the docs. Generally habtm is meant only for very very simple cases and if you start needing more complex things you should switch to has_many :through.

Olindaolinde

Answer 2

If you want to keep to the simple has_and_belongs_to_many association, you could add this:

class Person < ActiveRecord::Base
  has_and_belongs_to_many :posts
  before_destroy { posts.clear }
end

Which will clear the join table of all entries of that person. Note: This only removes records from the join table, it does not destroy the posts (which makes sense if it is a bi-direction has_and_belongs_to_many because the post might be referenced by other persons).

But guessing from your names (Person and Post) I would assume that you can probably get away with a Person has_many :posts and a Post belongs_to :person in which case you can use the :dependent => :destroy on the has_many association.

Betelgeuse

Could you please add how you would implement a dependent: :destroy action on a bi-directional has_and_belongs_to_many relationship which links to the same model? – Tog

I'm not sure if the syntax was ever originally like this answer, but I know in rails 4 it is supposed to be before_destroy { posts.clear } – Toomay

also you could use this to get the whole destroy effect after_destroy do User.find(:all, :uniq => true, :joins => :posts, :conditions => 'posts.id is NULL').each(&:destroy) end but might as well use has_many through at that point. – Marche

I found that i don't need to do before_destroy { posts.clear } in rails 4.2.0 (postgresql). Rails deletes relationship by default (it doesn't delete related record, post in this example). – Erato

Nice tip Faith. has_and_belongs_to_many continues to delete the associations in rails 5.0.0beta2 – Polemic

@Erato comment need to be an answer! – Beatabeaten

Answer 3

Try this:

class Person < ActiveRecord::Base
  has_and_belongs_to_many :posts
  before_destroy do
    posts.each { |post| post.destroy }
  end
end

You don't need to posts.clear as Rails 4.2+ handles this already.

Unstoppable

This is the correct answer. Rails 4.2+ and 5+ both will destroy association table as soon as you destroy the HABTM association. – Floozy