以下查询遇到为每条记录加载每个订单的"n+1"问题：
Job.joins('LEFT JOIN orders ON orders.job_id = jobs.id').order("orders.featured")
与此相同：
Job.includes(:order).order("orders.featured")
删除.order(…(部分会删除n+1问题，但它不会被排序。有什么办法解决这个问题吗？是否需要在父级中为"featured"属性创建一列？
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]]
尝试对相关数据使用预加载：
Job.joins('LEFT JOIN orders ON orders.job_id = jobs.id')
.order("orders.featured")
.preload(:orders)
Job.includes(:order).order("orders.featured")
Includes在视图中调用它之前不会将表连接在一起。它实际上将执行2个查询。如果您希望将这两个表连接起来以完成订单，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