ActiveRecord::CountLoader

N+1 count query killer for ActiveRecord.
ActiveRecord::CountLoader allows you to cache count of associated records by eager loading.

Why ActiveRecord::CountLoader?

Rails provides a way to resolve N+1 count query, which is belongs_to's counter_cache option.
It requires a column to cache the count. But adding a column just for count cache is overkill.

Thus this plugin enables you to preload counts in the same way as has_many and belongs_to.
count_loader is an ActiveRecord's association, which is preloadable by preload or includes.

Installation

Add this line to your application's Gemfile:

gem 'activerecord-count_loader'

Usage

Enable count_loader option

First, enable your has_many association's count_loader option.

 class Tweet
-  has_many :favorites
+  has_many :favorites, count_loader: true
 end

The option defines an additional association whose name is favorites_count.
Its association type is not an ordinary one (i.e. has_many, belongs_to) but count_loader.

Preload the association

This association works well by default.

@tweets = Tweet.all
@tweets.each do |tweet|
  p tweets.favorites_count # same as tweets.favorites.count
end

You can eagerly load count_loader association by includes or preload.

@tweets = Tweet.preload(:favorites_count)
@tweets.each do |tweet|
  p tweets.favorites_count # this line doesn't execute an additional query
end

Since it is association, you can preload nested count_loader association.

@users = User.preload(tweets: :favorites_count).all
@users.each do |user|
  user.tweets.each do |tweet|
    p tweet.favorites_count # this line doesn't execute an additional query
  end
end

Testing

$ bundle exec rake

Contributing

1. Fork it ( https://github.com/k0kubun/activerecord-count_loader/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request