predictive_load

Observes Active Record collections and notifies when a member loads an association. This allows for:

automatically preloading the association in a single query for all members of that collection.
N+1 detection logging

Automatic preloading

require 'predictive_load'
require 'predictive_load/active_record_collection_observation'
ActiveRecord::Base.send(:include, PredictiveLoad::ActiveRecordCollectionObservation)

require 'predictive_load/loader'

ActiveRecord::Relation.collection_observer = PredictiveLoad::Loader

Ticket.all.each do |ticket| 
  ticket.requester.identities.each { |identity| identity.account }
end

Produces:

  SELECT `tickets`.* FROM `tickets`
  SELECT `requesters`.* FROM `requesters` WHERE `requesters`.`id` IN (2, 7, 12, 32, 37)
  SELECT `identities`.* FROM `identities` WHERE `identities`.`requester_id` IN (2, 7, 12, 32, 37)
  SELECT `accounts`.* FROM `accounts` WHERE `accounts`.`id` IN (1, 2, 3)

Disabling preload

Some things cannot be preloaded, use predictive_load: false

has_many :foos, predictive_load: false

N+1 detection logging

There is also a log-only version:

require 'predictive_load'
require 'predictive_load/active_record_collection_observation'
ActiveRecord::Base.send(:include, PredictiveLoad::ActiveRecordCollectionObservation)

require 'predictive_load/watcher'

ActiveRecord::Relation.collection_observer = PredictiveLoad::Watcher

Comment.all.each do |comment|
  comment.account
end

Produces:

detected n1 call on Comment#account
expect to prevent 10 queries
would preload with: SELECT `accounts`.* FROM `accounts`  WHERE `accounts`.`id` IN (...)
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table    | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | accounts | const | PRIMARY       | PRIMARY | 4       | const |    10 |      |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
would have prevented all 10 queries

Known limitations:

Calling association#size will trigger an N+1 on SELECT COUNT(*). Work around by calling #length, loading all records.
Calling first / last will trigger an N+1.
Rails 4: unscoped will disable eager loading to circument a rails bug ... hopefully fixed in rails 5 https://github.com/rails/rails/pull/16531