Nested query in squeel | 易学教程

问题

Short version: How do I write this query in squeel?

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

Long version: rocket_tag is a gem that adds simple tagging to models. It adds a method tagged_with. Supposing my model is User, with an id and name, I could invoke User.tagged_with ['admin','sales']. Internally it uses this squeel code:

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

Which generates this query:

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

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

I believe a more agreeable way to write the query would be:

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

Is there any way to express this using squeel?

回答1:

I wouldn't know about Squeel, but the error you see could be fixed by upgrading PostgreSQL.

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

Starting with PostgreSQL 9.1, once you list a primary key in the GROUP BY you can skip additional columns for this table and still use them in the SELECT list. The release notes for version 9.1 tell us:

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

BTW, your alternative query can be simplified, an additional DISTINCT would be redundant.

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

来源：https://stackoverflow.com/questions/9763538/nested-query-in-squeel

标签

ruby-on-rails

ruby

postgresql

select

squeel