Undocumented options of doing eager loading in ActiveRecord

eager_loading.rb

# 1st way - do it automatically
# AR decides if it's better to use JOIN or separate query
User.includes(:profile)
# -> join or separate query

# 2nd way - always use JOIN
User.eager_load(:profile)
# -> SELECT `users`.`id` AS t0_r0, ..., `profiles`.`user_id` AS t1_r1, ... FROM `users` LEFT OUTER JOIN `profiles` ON `profiles`.`user_id` = `users`.`id`

# 3rd way - always do separate query
User.preload(:profile)
# -> SELECT * FROM users
# -> SELECT * FROM profiles WHERE user_id IN (...)

profile.rb

class Profile < ActiveRecord::Base
  belongs_to :user
end

user.rb

class User < ActiveRecord::Base
  has_one :profile
end