Question

My Rails 3 app has 2 models and a third that’s join table between them and their has_many relationships. Basically, User and Show are joined by SavedShow, allowing users to save a list of shows:

class Show < ActiveRecord::Base
  has_many :saved_shows
  has_many :users, :through => :saved_shows
end

class User < ActiveRecord::Base
  has_many :saved_shows
  has_many :shows, :through => :saved_shows
end

class SavedShow < ActiveRecord::Base
  belongs_to :user, :counter_cache => :saved_shows_count
  belongs_to :show
end

I’ve noticed that the counter_cache field (shows_saved_count) gets incremented automatically just fine, but not decremented. The core of the issue seems to be that removing shows from a user’s list is done via delete, which does not trigger updating of the counter_cache:

current_user.shows.delete(@show)

However, I can’t call the destroy method here, since that not only deleted the User/Show association in SavedShow, but also the Show object itself, which is not what I want.

Is a counter_cache in this kind of scenario not an appropriate use?

There appears to be a discussion about this as a bug back in 2009, and fixes were discussed, but I’m still seeing the issue in the latest Rails 3.0.

I would just write my own custom handling in the model, but there seems to be no after_delete callback that I can hook into (presumably this is the reason decrementing doesn’t work in the first place). Right now, there’s only one place in my own code where a delete of the association could occur, so I’ll just manually make a call to update the counter, but this seems like like such a fundamental shortcoming or bug of ActiceRecord associations with counter_cache, that I’m wondering if I’m not just missing something.

If this is indeed a genuine problem with counter_caches, what would be the best workaround?

Answer 1

Same issues here but on Rails 2.3.
Worth noticing that also adding a touch, like:

belongs_to :user, :counter_cache => :saved_shows_count, :touch => true

Won’t update counter cache nor the related updated_at field on association.delete(object).

To workaround the issue usually we manipulate the join model, but that also have some drawbacks.

Patch is here: https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/2824-patch-has_many-through-doesnt-update-counter_cache-on-join-model-correctly#ticket-2824-18