自参照协会：N + 1问题

Question

让说我有以下型号：

class Strategy < ActiveRecord::Base 
    belongs_to :user 
    has_many :snapshots, :class_name => 'Strategy', :foreign_key => 'master_id' 
    belongs_to :master, :class_name => 'Strategy', :counter_cache => :snapshots_count 

    scope :master_only, -> { where(:master_id => nil) } 
end 

因此，任何用户都可以创建的Strategy

主实例的快照在控制器我收到的所有“主” 的Strategy实例属于current_user

@strategies = current_user.strategies.master_only.includes(:user,:snapshots) 

滑轨正确地加载strategies和snapshots在两个查询而获取user在单独的查询每个快照因而引入（在此特定情况下N + 2）N + 1个问题：

Strategy Load (0.3ms) SELECT `strategies`.* FROM `strategies` WHERE strategies`.`user_id` = 2 AND `strategies`.`master_id` IS NULL LIMIT 10 OFFSET 0 
User Load (0.7ms) SELECT `users`.* FROM `users` WHERE `users`.`id` IN (2) 
Strategy Load (0.8ms) SELECT `strategies`.* FROM `strategies` WHERE `strategies`.`master_id` IN (56, 8, 55, 1, 58, 57, 24, 22) 
User Load (0.3ms) SELECT `users`.* FROM `users` WHERE `users`.`id` = 2 LIMIT 1 
....  
User Load (0.3ms) SELECT `users`.* FROM `users` WHERE `users`.`id` = 5 LIMIT 1 

有一种方法更有效地加载快照用户？

Answer 1

您可以对您有:user和:snapshots方式稍有变化，只生成一个查询加载所有属于:snapshots用户：

@strategies = current_user.strategies.master_only.includes(snapshots: :user) 

生成的SQL查询将是这样的：

SELECT `strategies`.* FROM `strategies` WHERE strategies`.`user_id` = 2 AND `strategies`.`master_id` IS NULL LIMIT 10 OFFSET 0 
SELECT `users`.* FROM `users` WHERE `users`.`id` IN (2) 
SELECT `strategies`.* FROM `strategies` WHERE `strategies`.`master_id` IN (56, 8, 55, 1, 58, 57, 24, 22) 
SELECT `users`.* FROM `users` WHERE `users`.`id` IN (2,5) 

UPDATE

根据OP，以下解决了N + 1问题：

@strategies = current_user.strategies.master_only.includes(:user, snapshots: :user)