ruby-on-rails - 如何使用 ActiveRelation 编写 UNION 链？

Question

我需要能够使用 ActiveRelation 将任意数量的子选择与 UNION 链接起来。

我对 ARel 的实现有点困惑，因为它似乎假设 UNION 是一个二进制运算。

但是:

( select_statement_a ) UNION ( select_statement_b ) UNION ( select_statement_c )

是有效的 SQL。这是否可以在不进行令人讨厌的字符串替换的情况下实现？

Answer 1

尽管亚当·拉塞克(Adam Lassek)走在正确的 rails 上，但你可以做得比他提出的更好一点。我刚刚解决了一个类似的问题，试图从社交网络模型中获取 friend 列表。可以通过各种方式自动获取好友，但我希望有一个 ActiveRelation 友好的查询方法来处理进一步的链接。所以我有

class User
    has_many :events_as_owner, :class_name => "Event", :inverse_of => :owner, :foreign_key => :owner_id, :dependent => :destroy
    has_many :events_as_guest, :through => :invitations, :source => :event

      def friends


        friends_as_guests = User.joins{events_as_guest}.where{events_as_guest.owner_id==my{id}}
        friends_as_hosts  = User.joins{events_as_owner}.joins{invitations}.where{invitations.user_id==my{id}}

        User.where do
          (id.in friends_as_guests.select{id}
          ) | 
          (id.in friends_as_hosts.select{id}
          )
        end
       end

end

它利用了 Squeels 子查询支持。生成的SQL是

SELECT "users".* 
FROM   "users" 
WHERE  (( "users"."id" IN (SELECT "users"."id" 
                           FROM   "users" 
                                  INNER JOIN "invitations" 
                                    ON "invitations"."user_id" = "users"."id" 
                                  INNER JOIN "events" 
                                    ON "events"."id" = "invitations"."event_id" 
                           WHERE  "events"."owner_id" = 87) 
           OR "users"."id" IN (SELECT "users"."id" 
                               FROM   "users" 
                                      INNER JOIN "events" 
                                        ON "events"."owner_id" = "users"."id" 
                                      INNER JOIN "invitations" 
                                        ON "invitations"."user_id" = 
                                           "users"."id" 
                               WHERE  "invitations"."user_id" = 87) )) 

通过对上述代码稍加修改来演示需要可变数量组件的替代模式

  def friends


    friends_as_guests = User.joins{events_as_guest}.where{events_as_guest.owner_id==my{id}}
    friends_as_hosts  = User.joins{events_as_owner}.joins{invitations}.where{invitations.user_id==my{id}}

    components = [friends_as_guests, friends_as_hosts]

    User.where do
      components = components.map { |c| id.in c.select{id} }
      components.inject do |s, i|
        s | i
      end
    end


  end

这是对OP确切问题的解决方案的粗略猜测

class Shift < ActiveRecord::Base
  def self.limit_per_day(options = {})
    options[:start]   ||= Date.today
    options[:stop]    ||= Date.today.next_month
    options[:per_day] ||= 5

    queries = (options[:start]..options[:stop]).map do |day|

      where{|s| s.scheduled_start >= day}.
      where{|s| s.scheduled_start < day.tomorrow}.
      limit(options[:per_day])

    end

    where do
      queries.map { |c| id.in c.select{id} }.inject do |s, i|
        s | i
      end
    end
  end
end