ActiveRecord 限制从 company_ids 集合返回的用户

Question

I have an array of company_ids
In the User model, I would like to find_by company_ids, but for each company_id, the user list returned should not exceed 5 users. How can I do that.

I have tried doing this

User.where(company_id: [1, 2, 3]).group('users.company_id').having('count(company_id) <= 5')

But this will return only those set where users in a company are not more than 5. Moreover, this wont return all the records.

What I am trying to do is, for company_ids = [1, 2, 3] , the collection should be able to return at max 5 users for company_id 1, then company_id 2, then company_id 3, and not load all the users for all those company_ids.

Answer 1

I think the further you can due to your MySQL version is just using UNION ALL .

You can create a query selecting the users by their company_id and then applying the LIMIT :

(select * from users where company_id = 1 LIMIT 2)
UNION ALL
(select * from users where company_id = 2 LIMIT 2)
UNION ALL
(select * from users where company_id = 3 LIMIT 2)

As Rails doesn't have a built-in support for UNION queries, you can create your own statement:

sql = [1, 2, 3].map { |id| "(SELECT * FROM users WHERE company_id = #{id} LIMIT 2)" }
               .join(" UNION ALL ")
User.find_by_sql(sql)

---

Update using MySQL8+

SELECT id, name, company_id
FROM (
  SELECT id, name, company_id, ROW_NUMBER() OVER w AS company_user
  FROM users
  WINDOW w AS (PARTITION BY company_id)
) users
WHERE company_user <= 2;

The ActiveRecord way would be:

User
  .select(:id, :name, :company_id)
  .from(
    User.select(:id, :name, :company_id, 'ROW_NUMBER() OVER w AS company_user')
        .from('users WINDOW w AS (PARTITION BY company_id)')
  )
  .where('company_user <= 2')