Question

我正在尝试优化Active Record中查询的性能。以前，我会执行两个SQL查询，并且应该可以一次执行。

这些是我正在其中运行查询的表：

# Table name: notifications
#
#  id         :integer          not null, primary key
#  content    :text(65535)
#  position   :integer

# Table name: dismissed_notifications
#
#  id              :integer          not null, primary key
#  notification_id :integer
#  user_id         :integer

这是现有查询：

where.not(id: user.dismissed_notifications.pluck(:id))

产生：

SELECT `dismissed_notifications`.`id` FROM `dismissed_notifications` WHERE `dismissed_notifications`.`user_id` = 655

SELECT  `notifications`.* FROM `notifications` WHERE (`notifications`.`id` != 1)

这是我想获取的SQL，它返回相同的记录：

select *
from notifications n
where not exists(
    select 1
    from dismissed_notifications dn
    where dn.id = n.id
      and dn.user_id = 655)

Answer 1

您可以像下面这样编写not exists查询

where('NOT EXISTS (' + user.dismissed_notifications.where('dismissed_notifications.id = notifications.id').to_sql + ')')

OR

还有另一种减少查询数量的方法是使用select代替pluck，它将创建子查询而不是从数据库中提取记录。 Rails ActiveRecord Subqueries

where.not(id: user.dismissed_notifications.select(:id))

这将在下面生成SQL查询

SELECT  `notifications`.* 
  FROM `notifications` 
  WHERE (
    `notifications`.`id` NOT IN 
      (SELECT `dismissed_notifications`.`id` 
        FROM `dismissed_notifications` 
        WHERE `dismissed_notifications`.`user_id` = 655
      )
  )

如何使用Active Record使用SQL

1 个答案: