选择记录根据具体的情况从另一个表

Question

在我正在工作的Rails应用程序，我有以下设置：

class Volunteer < AR::Base 
    has_many :engagements 
end 

class Engagement < AR::Base 
    belongs_to :volunteer 
end 

我想选择谁不是在某一特定日期从事所有的志愿者或日期范围之间：

scope :not_engaged_on, lambda { |q| joins(:engagements).where(["engagements.date != ?", q])} 

scope :not_engaged_between, lambda { |s, e| joins(:engagements).where(["engagements.date NOT BETWEEN ? AND ?", s, e]) } 

这些范围我正与其他范围合并来过滤志愿者。

以上范围:not_engaged_between产生以下SQL：

SELECT "volunteers".* FROM "volunteers" INNER JOIN "accounts" ON "accounts"."user_id" = "volunteers"."id" AND "accounts"."user_type" = 'Volunteer' INNER JOIN "engagements" ON "engagements"."volunteer_id" = "volunteers"."id" WHERE (accounts.activation_state = 'active') AND (engagements.date NOT BETWEEN '2012-07-30' AND '2012-08-04') 

的问题是志愿者可能没有任何订婚到现在，并在这种情况下，查询完全不理会那些志愿者。即使他们没有任何交往，我如何能够让所有志愿者回归？

Answer 1

你需要一个not exists来做到这一点。这个语法在SQL上有点沉重，也许有更好的办法来做到这一点。

scope :not_engaged_on, lambda { |q| where(["NOT EXISTS (SELECT ID from engagements WHERE engagements.date = ? AND volunteer_id = volunteers.id)", q])} 

scope :not_engaged_between, lambda { |s, e| where(["NOT EXISTS (SELECT ID from engagements WHERE engagements.date BETWEEN ? AND ? AND volunteer_id = volunteers.id)", s, e]) } 

Answer 2

@Anthony已经提到NOT EXISTS。另一种选择是一个LEFT [OUTER] JOIN +检查它是否存在（e.volunteer_id IS NULL）：

SELECT v.* 
FROM volunteers  v 
JOIN accounts   a ON a.user_id = v.id 
LEFT JOIN engagements e ON e.volunteer_id = v.id AND e.date BETWEEN '2012-07-30' AND '2012-08-04' 
WHERE a.user_type = 'Volunteer' 
AND a.activation_state = 'active' 
AND e.volunteer_id IS NULL; 

你必须条件e.date BETWEEN '2012-07-30' AND '2012-08-04'移动到在这种情况下连接语句。

对于accounts使用另一个LEFT JOIN，如果它也可能丢失。但看来你确实想要检查user_type和activation_state。