ruby-on-rails - squeel 中的嵌套查询

Question

简短版:如何在 squeel 中编写此查询？

SELECT OneTable.*, my_count
FROM   OneTable JOIN (
  SELECT DISTINCT one_id, count(*) AS my_count
  FROM   AnotherTable
  GROUP BY one_id
) counts
ON OneTable.id=counts.one_id

---

长版:rocket_tag是一个向模型添加简单标记的 gem。它添加了一个方法 tagged_with。假设我的模型是 User，具有 ID 和名称，我可以调用 User.tagged_with ['admin','sales']。在内部它使用这个 squeel 代码:

select{count(~id).as(tags_count)}
.select("#{self.table_name}.*").
joins{tags}.
where{tags.name.in(my{tags_list})}.
group{~id}

生成此查询:

SELECT count(users.id) AS tags_count, users.*
  FROM users INNER JOIN taggings
    ON taggings.taggable_id = users.id
   AND taggings.taggable_type = 'User'
  INNER JOIN tags
    ON tags.id = taggings.tag_id
  WHERE tags.name IN ('admin','sales')
  GROUP BY users.id

一些 RDBMS 对此很满意，但 postgres 提示:

ERROR: column "users.name" must appear in the GROUP BY
clause or be used in an aggregate function

我认为编写查询的更合适的方式是:

SELECT users.*, tags_count FROM users INNER JOIN (
  SELECT DISTINCT taggable_id, count(*) AS tags_count
    FROM taggings INNER JOIN tags
      ON tags.id = taggings.tag_id
    WHERE tags.name IN ('admin','sales')
    GROUP BY taggable_id
  ) tag_counts
  ON users.id = tag_counts.taggable_id

有什么方法可以用 squeel 表达吗？

Answer 1

我不知道 Squeel，但您看到的错误可以通过升级 PostgreSQL 来修复。

Some RDBMSs are happy with this, but postgres complains:

ERROR: column "users.name" must appear in the GROUP BY clause or be used in an aggregate function

从 PostgreSQL 9.1 开始，一旦您在 GROUP BY 中列出了一个主键，您就可以跳过该表的其他列，并仍然在 SELECT 列表中使用它们。 release notes for version 9.1告诉我们:

Allow non-GROUP BY columns in the query target list when the primary key is specified in the GROUP BY clause

---

顺便说一句，您的替代查询可以简化，额外的 DISTINCT 将是多余的。

SELECT o.*, c.my_count
FROM   onetable o
JOIN (
  SELECT one_id, count(*) AS my_count
  FROM   anothertable
  GROUP  BY one_id
) c ON o.id = counts.one_id