Rails counter_cache not updating correctly

Question

Using Rails 3.1.3 and I\'m trying to figure out why our counter caches aren\'t being updated correctly when changing the parent record id via update_attributes.

Answer 1

From the fine manual:

:counter_cache

Caches the number of belonging objects on the associate class through the use of increment_counter and decrement_counter. The counter cache is incremented when an object of this class is created and decremented when it’s destroyed.

There's no mention of updating the cache when an object is moved from one owner to another. Of course, the Rails documentation is often incomplete so we'll have to look at the source for confirmation. When you say :counter_cache => true, you trigger a call to the private add_counter_cache_callbacks method and add_counter_cache_callbacks does this:

1. Adds an after_create callback which calls increment_counter.
2. Adds an before_destroy callback which calls decrement_counter.
3. Calls attr_readonly to make the counter column readonly.

I don't think you're expecting too much, you're just expecting ActiveRecord to be more complete than it is.

All is not lost though, you can fill in the missing pieces yourself without too much effort. If you want to allow reparenting and have your counters updated, you can add a before_save callback to your ExhibitorRegistration that adjusts the counters itself, something like this (untested demo code):

class ExhibitorRegistration < ActiveRecord::Base
  belongs_to :event, :counter_cache => true
  before_save :fix_counter_cache, :if => ->(er) { !er.new_record? && er.event_id_changed? }

private

  def fix_counter_cache
    Event.decrement_counter(:exhibitor_registration_count, self.event_id_was)
    Event.increment_counter(:exhibitor_registration_count, self.event_id)
  end

end

If you were adventurous, you could patch something like that into ActiveRecord::Associations::Builder#add_counter_cache_callbacks and submit a patch. The behavior you're expecting is reasonable and I think it would make sense for ActiveRecord to support it.