Find_by_sql作为Rails范围

来自Sitepoint的r937非常友好，可以帮助我找出从数据库返回正确结果所需的查询。

我需要的是能够将此查询用作范围，并能够将其他范围链接到此范围。

查询是：

SELECT coasters.* FROM ( SELECT order_ridden, MAX(version) AS max_version FROM coasters GROUP BY order_ridden ) AS m INNER JOIN coasters ON coasters.order_ridden = m.order_ridden AND COALESCE(coasters.version,0) = COALESCE(m.max_version,0) 

我尝试制作这样的范围：

  scope :uniques, lambda { find_by_sql('SELECT coasters.* FROM ( SELECT order_ridden, MAX(version) AS max_version FROM coasters GROUP BY order_ridden ) AS m INNER JOIN coasters ON coasters.order_ridden = m.order_ridden AND COALESCE(coasters.version,0) = COALESCE(m.max_version,0)') } 

但是当我尝试将另一个示波器链接到它上面时，它失败了。 有没有办法像普通范围一样运行此查询？

Answer 1

find_by_sql返回一个Array 。 但是你需要一个ActiveRecord::Relation来链接其他范围。

使用将返回ActiveRecord::Relation ActiveRecord方法重写查询的一种方法是重新排列它，以便嵌套发生在INNER JOIN部分。

你可能想尝试类似的东西：

 scope :uniques, lambda { max_rows = select("order_ridden, MAX(version) AS max_version").group(:order_ridden) joins("INNER JOIN (#{max_rows.to_sql}) AS m ON coasters.order_ridden = m.order_ridden AND COALESCE(coasters.version,0) = COALESCE(m.max_version,0)") }