基於計數的更新

Question

我想基於poll_records表更新users表中的number_of_votes。在軌道上，它看起來像

User.all.each do |user| 
    user.number_of_votes = user.poll_records.where("poll_record_type = 3").size 
    user.save 
end 

我是PostgreSQL的新手。我如何以這種方式更新用戶？

Answer 1

可能是這樣的：

UPDATE users u 
SET number_of_votes = p.ct 
FROM (
    SELECT users_id, count(*) AS ct 
    FROM poll_records 
    WHERE poll_record_type = 3 
    GROUP BY users_id 
    ) p 
WHERE u.users_id = p.users_id 
AND  u.number_of_votes IS DISTINCT FROM p.ct; -- to avoid empty updates 

我不會用（保留字）或混合大小寫標識符作爲表名。它強制雙引號。