:include vs :joins

class User; has_many :leave_credits

then User.where(:id => [1,2]).joins(:leave_credits).count

# => 8 # number of leave_credits.
# SELECT COUNT(*) FROM `users` INNER JOIN `leave_credits` ON `leave_credits`.`user_id` = `users`.`id` WHERE `users`.`id` IN (1, 2)
# may lead to 1+n problem

then User.where(:id => [1,2]).includes(:leave_credits).count

# => 2 # number of users.
# SELECT COUNT(*) FROM `users` WHERE `users`.`id` IN (1,2)
# will not query projects until you call user.leave_credits
# avoids 1+n problem

Other infos: Railscast #181

Validation

Triggers validation:

• create
• create!
• save
• save!
• update
• update_attributes
• update_attributes!

Structure