Rails has_many through polymorphic counter cache

activerecord, counter-cache, has-many-through, polymorphism, ruby-on-rails

Solution

Simply modify your Classification model for the following:

class Classification < ActiveRecord::Base
  attr_accessible :classifiable, :classifiable_id, :classifiable_type,
                  :taxonomy, :taxonomy_id, :taxonomy_type

  belongs_to :classifiable, :polymorphic => true
  belongs_to :taxonomy,     :polymorphic => true

  before_create  :increment_counter
  before_destroy :decrement_counter

  private

  # increments the right classifiable counter for the right taxonomy
  def increment_counter
    self.taxonomy_type.constantize.increment_counter("#{self.classifiable_type.downcase.pluralize}_count", self.taxonomy_id)
  end

  # decrements the right classifiable counter for the right taxonomy
  def decrement_counter
    self.taxonomy_type.constantize.decrement_counter("#{self.classifiable_type.downcase.pluralize}_count", self.taxonomy_id)
  end
end

Also, make sure you have the following columns in your taxonomies table:

t.integer :questions_count,           :null => false, :default => 0
t.integer :other_classifiables_count, :null => false, :default => 0
t.integer :other_classifiables_count, :null => false, :default => 0
t.integer :other_classifiables_count, :null => false, :default => 0

Change "other_classifiables_count" to what you need ("answers_count", "users_count", etc.)

Problem

I have two models I link together using a polymorphic has_many through association and I would like to add a counter_cache but it seems Rails/ActiveRecord does not support this feature out of the box. ``` class Classifiable < ActiveRecord::Base has_many :classifications, :as => :classifiable, :foreign_key => :classifiable_id end class Taxonomy < ActiveRecord::Base has_many :classifications, :as => :taxonomy, :foreign_key => :taxonomy_id end class Question < Classifiable has_many :categories, :through => :classifications, :as => :classifiable, :source => :taxonomy, :source_type => "Category" end class Category < Taxonomy has_many :questions, :through => :classifications, :source => :classifiable, :source_type => "Question" end class Classification < ActiveRecord::Base attr_accessible :classifiable, :classifiable_id, :classifiable_type, :taxonomy, :taxonomy_id, :taxonomy_type belongs_to :classifiable, :polymorphic => true belongs_to :taxonomy, :polymorphic => true end ```

Original source