Why doesn't collection=objects on has_many :through trigger destroy callbacks on the join model?

[spike22]

I've asked this on on Stack Overflow but didn't receive much of a response:

The Rails 4 documentation says this regarding destroy callbacks on the join model for a has_many :through relationship:

collection=objects Replaces the collections content by deleting and adding objects as appropriate. If the :through option is true callbacks in the join models are triggered except destroy callbacks, since deletion is direct.

Thankfully it's documented at least, but I want to know why on earth this is the case? It makes more sense to trigger destroy callbacks (or have the option to) on a :through since these types of models can have destroy callbacks and other associations.

In my case I had a has_and_belongs_to_many relationship on the join tables model off to another model. The records on that second join table would never be deleted when the associated records on the first join table were deleted. I resorted to this which feels hacky, and I have to repeat myself on each side of the :through relationship:

class SchoolsTemplate < ActiveRecord::Base

  belongs_to :school
  belongs_to :template

  has_and_belongs_to_many :groups

end




class School < ActiveRecord::Base

  has_many :schools_templates, dependent: :destroy
  has_many :templates, through: :schools_templates, before_remove: :remove_groups_school_templates

  private

  def remove_groups_school_templates(template)
    schools_templates.where(template: template).first.groups.clear
  end

end

There's a validation to 'ensure' uniqueness on the join tables records between the two foreign keys, so that's why I can call first in the callback.

[Eric Krause]

I responded on stackoverflow, but I'll do here as well so that it is covered everywhere on the internet.

In my case I was doing something similar to what you were doing and was running into the same issue with the join table being deleted instead of destroyed.

I started looking through the code and I believe the documentation is just out of date.

[has_many_through_association](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/associations/has_many_through_association.rb#L131)

All you need to do is add the dependent: :destroy to the has_many :through relationship.

    class User

      has_many :partnerships, dependent: :destroy

      has_many :partners, through: :partnerships, dependent: :destroy

    end

The pain I was dealing with was:

    

    user.partner_ids = [1,2,3]

    #creates the relationships

    user.partner_ids = []

    #was deleting the records from partnerships without callbacks.

The dependent: :destroy on the partners relationship fixed that.  Callbacks are now being run and things are good again.

Eric

[spike22]

Hi Eric, that's really interesting! I'll give it a whirl and report back :) I might do a documentation pull request to clarify this too.

Have a great day!

Brendon

[Eric Krause]

I just did that this morning. 

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Eric Krause

[spike22]

Haha! very good :)

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Eric Krause

[spike22]

Just reporting back, it works as expected on my end too. Thanks for the feedback. This solution feels more proper :)

Linking in the PR for the documentation change for others finding this thread in the future: https://github.com/rails/rails/pull/22644

Cheers,

Brendon