Sitepoint的r937足以帮助我确定需要从数据库返回正确结果的查询。
我需要的是能够将此查询用作作用域,并能够将其他作用域链接到该作用域。
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)
我试图使像这样的范围:
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时,它失败了。有没有办法像正常作用域一样运行此查询?
最佳答案
find_by_sql
返回一个Array
。但是,您需要一个ActiveRecord::Relation
来链接其他作用域。
一种使用ActiveRecord方法重写查询以返回ActiveRecord::Relation
的方法是将其重新排列一点,以使嵌套发生在INNER JOIN
部分中。
您可能想尝试类似的方法:
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)")
}
关于sql - Find_by_sql作为Rails范围,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/25454696/