如何获得 Active Record 中子查询的计数? [英] How do you get a count of a subquery in Active Record?

问题描述

我正在将 SQL 查询迁移到 Active Record.这是一个简化的版本:

I'm migrating an SQL query to Active Record. Here is a simplified version:

SELECT count(*) FROM (
  SELECT type, date(created_at)
  FROM notification_messages
  GROUP BY type, date(created_at)
) x

我不确定如何在 Active Record 中实现这一点.这可行，但很混乱:

I'm not sure how to implement this in Active Record. This works, but it's messy:

sql = NotificationMessage.
  select("type, date(created_at) AS period").
  group("type", "period").
  to_sql
NotificationMessage.connection.exec_query("SELECT count(*) FROM (#{sql}) x")

另一种可能性是在 Ruby 中进行计数，但效率会降低:

Another possibility is to do the count in Ruby, but that would be less efficient:

NotificationMessage.
  select("type, date(created_at) AS period").
  group("type", "period").
  length

有没有更好的解决方案?

Is there a better solution?

推荐答案

Rails 有 from 方法.所以我会写它:

Rails has the from method. So I would write it:

NotificationMessage
  .from(
    NotificationMessage
      .select("type, date(created_at)")
      .group("type, date(created_at)"), :x
  ).count

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