ActiveRecord::Precount

N+1 count query killer for ActiveRecord. Yet another counter_cache alternative.
ActiveRecord::Precount allows you to cache count of associated records by eager loading.

Synopsis

N+1 count query

Sometimes you may see many count queries for one association. You can use counter_cache to solve it, but it costs much to use counter_cache.

Tweet.all.each do |tweet|
  p tweet.favorites.count
end
# SELECT `tweets`.* FROM `tweets`
# SELECT COUNT(*) FROM `tweets` WHERE `tweets`.`tweet_id` = 1
# SELECT COUNT(*) FROM `tweets` WHERE `tweets`.`tweet_id` = 2
# SELECT COUNT(*) FROM `tweets` WHERE `tweets`.`tweet_id` = 3
# SELECT COUNT(*) FROM `tweets` WHERE `tweets`.`tweet_id` = 4
# SELECT COUNT(*) FROM `tweets` WHERE `tweets`.`tweet_id` = 5

Count eager loading

With activerecord-precount gem installed, you can use precount method to eagerly load counts of associated records.

Tweet.all.precount(:favorites).each do |tweet|
  p tweet.favorites.count
end
# SELECT `tweets`.* FROM `tweets`
# SELECT COUNT(`tweets`.`tweet_id`), tweet_id FROM `tweets` WHERE `tweets`.`tweet_id` IN (1, 2, 3, 4, 5) GROUP BY tweet_id

Benchmark

With this benchmark (result), precounted query is 7.7x faster than N+1 count query.

# Tweet count is 50, and each tweet has 10 favorites
Tweet.precount(:favorites).first(50).map(&:favorites_count) # 0.190
Tweet.first(50).map{ |t| t.favorites.count }                # 1.472

Installation

Add this line to your application's Gemfile:

gem 'activerecord-precount'

Supported Versions

• Ruby
  • 2.0, 2.1, 2.2
• Rails
  • 4.1, 4.2
• Databases
  • sqlite
  • mysql
  • postgresql

Advanced Usage

Nested eager loading

Foo.precount(:bars) automatically defines bars_count association for Foo. Thus you can preload the association and call foo.bars_count.

You can manually define bars_count with follwoing code.

 class Foo < ActiveRecord::Base
-  has_many :bars
+  has_many :bars, count_loader: true
 end

Then there are two different ways to preload the bars_count.

# the same
Foo.preload(:bars_count)
Foo.precount(:bars)

With this condition, you can eagerly load nested association by preload.

Hoge.preload(foo: :bars_count)

Performance issue

With activerecord-precount gem installed, bars.count fallbacks to bars_count if bars_count is defined. Though precounted bars.count is faster than not-precounted one, the fallback is currently much slower than just calling bars_count.

# slow
Foo.precount(:bars).map { |f| f.bars.count }

# fast (recommended)
Foo.precount(:bars).map { |f| f.bars_count }

License

MIT License