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This is a simple approach to creating a custom counter cache that can be used across multiple models.

In Rails 2.3, add the following to your environment.rb:

config.gem 'custom_counter_cache'

If using Bundler, add the following to your Gemfile:

gem 'custom_counter_cache'

This is the block that will be used to calculate the value for the counter cache. It will be called by other models through their association via an after_save or after_destroy callback.

define_counter_cache :articles_count do |user|
  user.articles.where(state: 'published').count
end

This will define the after_create, after_update and after_destroy callbacks. An :if option can be provided to limit when these callbacks get triggered.

update_counter_cache :user, :articles_count, if: -> (article) { article.state_changed? }

These callbacks can be added to any number of models that might need to change the counter cache.

To store the counter cache you need to create a column for the model with the counter cache (example: articles_count).

If you would like to store all of your counter caches in a single table, you can use this migration:

create_table :counters do |t|
  t.references :countable, polymorphic: true
  t.string :key, null: false
  t.integer :value, null: false, default: 0
  t.timestamps
end
add_index :counters, [ :countable_id, :countable_type, :key ], unique: true

Here is the example model to go with:

class Counter < ActiveRecord::Base
  belongs_to :countable, polymorphic: true
  validates :countable, presence: true
end

If you would like to store your counter cache in an existing table, you can use this migration:

def change
  add_column :users, :articles_count, :integer, default: 0, null: false
end

To backfill your counters, run something like this either in a migration or in the console:

User.select(:id).find_each(batch_size: 100) { |u| u.update_articles_count }

(The MIT License)

Copyright © 2011-2015 Cedric Howe

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the ‘Software’), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

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