Rails :counter_cache is not triggering after_update callback

Question

So, I have a Folder and a FolderItem models.

UPDATE

# == Schema Information
#
# Table name: folders
#
#  id                 :integer         not null, primary key
#  name               :string(255)     not null
#  parent_folder_id   :integer
#  user_id            :integer         not null
#  folder_itens_count :integer         default(0)
#  created_at         :datetime
#  updated_at         :datetime
#

class Folder < ActiveRecord::Base
...

  belongs_to :parent_folder, :class_name => 'Folder'
  has_many :child_folders, :class_name => 'Folder', :foreign_key => :parent_folder_id
  has_many :folder_itens, :order => 'created_at DESC'

  after_update {
    update_parent_folder_itens_count
  }

  def update_parent_folder_itens_count
    parent_folder = self.parent_folder
    if self.folder_itens_count_changed? && parent_folder
      quant_changed = self.folder_itens_count - self.folder_itens_count_was
      parent_folder.increment(:folder_itens_count, quant_changed)
    end
  end
end

class FolderItem < ActiveRecord::Base
... 
  belongs_to :folder, :counter_cache => :folder_itens_count
end

I'm using a counter_cache to keep the number of itens of a single folder. But a folder might be the parent of another folder, and I wanted the parent folder to have the sum of counter_cache of all it's children plus it's own counter_cache.

To do so I tried to put a after_update method caching the changes made in the counter_cache column, but somehow this method is not being called when a new FolderItem is created.

Answer 1

I'd do something like this.

Add some cache counter fields to folders table

$ rails g migration add_cache_counters_to_folders child_folders_count:integer \
                                                  folder_items_count:integer  \
                                                  total_items_count:integer   \
                                                  sum_of_children_count:integer

And Ruby code

class Folder < ActiveRecord::Base
  belongs_to :parent_folder, class_name: 'Folder', counter_cache: :child_folders_count
  has_many :child_folders, class_name: 'Folder', foreign_key: :parent_folder_id
  has_many :folder_items

  before_save :cache_counters

  # Direct descendants - files and items within this folder
  def total_items
    child_folders_count + folder_items_count
  end

  # All descendants - files and items within all the folders in this folder
  def sum_of_children
    folder_items_count + child_folders.map(&:sum_of_children).inject(:+)
  end

private
  def cache_counters
    self.total_items_count = total_items
    self.sum_of_children_count = sum_of_children
  end
end

class FolderItem < ActiveRecord::Base
  belongs_to :folder, counter_cache: true # folder_items_count
end

Please note that the Folder#sum_of_children-method is recursive, so for large datasets it may slow down your application. You may want to do some more SQL-magic to it, but as pure Ruby this is as close to a good solution as I can get. I saw you did it the other way around, that will be just as slow as you need to update from the bottom-up as well. (This is top-down)

Don't know if that's what you're looking for, but it's a readable solution for caching the number of items within a folder.