Counter cache for HABTM

on_pry.rb

> Rails.env
# => "test"

 > c1,c2,c3 = Category.make!,Category.make!,Category.make!
[deprecated] I18n.enforce_available_locales will default to true in the future. If you really want to skip validation of your locale you can set I18n.enforce_available_locales = false to avoid this message.
   (0.4ms)  BEGIN
test
  SQL (0.5ms)  INSERT INTO `categories` (`created_at`, `description`, `name`, `updated_at`) VALUES ('2014-01-01 04:08:14', 'syndicate strategic web-readiness', 'Books', '2014-01-01 04:08:14')
   (1.9ms)  COMMIT
  Category Load (0.3ms)  SELECT `categories`.* FROM `categories` WHERE `categories`.`id` = 16 LIMIT 1
   (0.1ms)  BEGIN
test
  SQL (0.2ms)  INSERT INTO `categories` (`created_at`, `description`, `name`, `updated_at`) VALUES ('2014-01-01 04:08:14', 'enhance clicks-and-mortar bandwidth', 'Electronics & Computers', '2014-01-01 04:08:14')
   (0.8ms)  COMMIT
  Category Load (0.2ms)  SELECT `categories`.* FROM `categories` WHERE `categories`.`id` = 17 LIMIT 1
   (0.1ms)  BEGIN
test
  SQL (0.1ms)  INSERT INTO `categories` (`created_at`, `description`, `name`, `updated_at`) VALUES ('2014-01-01 04:08:14', 'leverage front-end synergies', 'Home, Garden & Tools', '2014-01-01 04:08:14')
   (0.3ms)  COMMIT
  Category Load (0.1ms)  SELECT `categories`.* FROM `categories` WHERE `categories`.`id` = 18 LIMIT 1
  

> website = Website.make!(categories: [c1, c2, c3])
  SQL (0.7ms)  UPDATE `categories` SET `websites_count` = COALESCE(`websites_count`, 0) + 1 WHERE `categories`.`id` = 16
  SQL (0.4ms)  UPDATE `categories` SET `websites_count` = COALESCE(`websites_count`, 0) + 1 WHERE `categories`.`id` = 17
  SQL (0.5ms)  UPDATE `categories` SET `websites_count` = COALESCE(`websites_count`, 0) + 1 WHERE `categories`.`id` = 18
   (0.2ms)  BEGIN
  SQL (0.3ms)  INSERT INTO `websites` (`created_at`, `description`, `likes_count`, `name`, `updated_at`, `url`) VALUES ('2014-01-01 04:08:17', 'engage back-end metrics', 48486642, 'Elmer Pfeffer', '2014-01-01 04:08:17', 'http://runolfonprohaska.name')
   (0.1ms)  INSERT INTO `categories_websites` (`website_id`, `category_id`) VALUES (37, 16)
   (0.1ms)  INSERT INTO `categories_websites` (`website_id`, `category_id`) VALUES (37, 17)
   (0.1ms)  INSERT INTO `categories_websites` (`website_id`, `category_id`) VALUES (37, 18)
   (0.3ms)  COMMIT
  Website Load (0.2ms)  SELECT `websites`.* FROM `websites` WHERE `websites`.`id` = 37 LIMIT 1
=> #<Website id: 37, name: "Elmer Pfeffer", description: "engage back-end metrics", url: "http://runolfonprohaska.name", likes_count: 48486642, comments_count: 0, created_at: "2014-01-01 04:08:17", updated_at: "2014-01-01 04:08:17", logo_file_name: nil, logo_content_type: nil, logo_file_size: nil, logo_updated_at: nil>


> website.categories = [c1, c2]
  Category Load (0.4ms)  SELECT `categories`.* FROM `categories` INNER JOIN `categories_websites` ON `categories`.`id` = `categories_websites`.`category_id` WHERE `categories_websites`.`website_id` = 37
   (0.1ms)  BEGIN
   (0.2ms)  DELETE FROM `categories_websites` WHERE `categories_websites`.`website_id` = 37 AND `categories_websites`.`category_id` IN (18)
  SQL (0.1ms)  UPDATE `categories` SET `websites_count` = COALESCE(`websites_count`, 0) - 1 WHERE `categories`.`id` = 18
   (1.5ms)  COMMIT
=> [#<Category id: 16, name: "Books", description: "syndicate strategic web-readiness", created_at: "2014-01-01 04:08:14", updated_at: "2014-01-01 04:08:14", websites_count: nil>,
 #<Category id: 17, name: "Electronics & Computers", description: "enhance clicks-and-mortar bandwidth", created_at: "2014-01-01 04:08:14", updated_at: "2014-01-01 04:08:14", websites_count: nil>]
 
 

Website.rb

class Website < ActiveRecord::Base
  has_and_belongs_to_many :categories, after_add: :increment_categories_counter_cache, after_remove: :decrement_categories_counter_cache

  private

  def increment_categories_counter_cache(category)
    Category.increment_counter(:websites_count, category.id )
  end

  def decrement_categories_counter_cache(category)
    Category.decrement_counter(:websites_count, category.id )
  end
end