Question

以下查询遇到加载每条记录的每个订单的“n + 1”问题：

Job.joins('LEFT JOIN orders ON orders.job_id = jobs.id').order("orders.featured")

同样如此：

Job.includes(:order).order("orders.featured")

删除.order（...）部分会删除n + 1问题，但后来没有订购。任何想法如何解决这一问题？我是否需要在父级中为'featured'属性创建一个列？

输出：

  Order Load (0.4ms)  SELECT  "orders".* FROM "orders" WHERE "orders"."job_id" = $1 LIMIT 1  [["job_id", 26]]
  Rendered jobs/_job.html.erb (1.9ms)
  Order Load (0.3ms)  SELECT  "orders".* FROM "orders" WHERE "orders"."job_id" = $1 LIMIT 1  [["job_id", 3]]
  Rendered jobs/_job.html.erb (2.0ms)
  Order Load (0.3ms)  SELECT  "orders".* FROM "orders" WHERE "orders"."job_id" = $1 LIMIT 1  [["job_id", 52]]
  Rendered jobs/_job.html.erb (1.7ms)
  Order Load (0.3ms)  SELECT  "orders".* FROM "orders" WHERE "orders"."job_id" = $1 LIMIT 1  [["job_id", 13]]
  Rendered jobs/_job.html.erb (1.9ms)
  Order Load (0.3ms)  SELECT  "orders".* FROM "orders" WHERE "orders"."job_id" = $1 LIMIT 1  [["job_id", 34]]
  Rendered jobs/_job.html.erb (1.9ms)
  Order Load (0.4ms)  SELECT  "orders".* FROM "orders" WHERE "orders"."job_id" = $1 LIMIT 1  [["job_id", 64]]
  Rendered jobs/_job.html.erb (2.8ms)
  Order Load (0.4ms)  SELECT  "orders".* FROM "orders" WHERE "orders"."job_id" = $1 LIMIT 1  [["job_id", 94]]
  Rendered jobs/_job.html.erb (3.2ms)
  Order Load (0.4ms)  SELECT  "orders".* FROM "orders" WHERE "orders"."job_id" = $1 LIMIT 1  [["job_id", 60]]
  Rendered jobs/_job.html.erb (3.1ms)
  Order Load (0.4ms)  SELECT  "orders".* FROM "orders" WHERE "orders"."job_id" = $1 LIMIT 1  [["job_id", 29]]

Answer 1

尝试将preload用于关联数据：

Job.joins('LEFT JOIN orders ON orders.job_id = jobs.id')
  .order("orders.featured")
  .preload(:orders)

Answer 2

Job.includes(:order).order("orders.featured")

包括不将表连接在一起，直到您在视图中调用它为止。它实际上会做2个查询。如果您希望连接2个表来执行订单，则需要使用eager_load：

Job.eager_load(:order).order("orders.featured")

http://blog.bigbinary.com/2013/07/01/preload-vs-eager-load-vs-joins-vs-includes.html

按孩子的属性排序时n + 1问题

2 个答案: