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我有一种情况，学校和 EventLeg 记录通过属于给定事件的照片绑定在一起。

class Photo < ActiveRecord::Base
  belongs_to :event
  belongs_to :event_leg
  has_and_belongs_to_many :schools
end
class Event < ActiveRecord::Base
  has_many :event_legs, :through => :photos, :group => 'event_legs.id'
  has_many :photos
end
class School < ActiveRecord::Base
  has_and_belongs_to_many :photos
  has_many :events, :through => :photos
  has_many :event_legs, :through => :photos
end
class EventLeg < ActiveRecord::Base
  has_many :photos
  has_many :schools, :through => :photos
end

我需要在某个事件中获取出现在给定事件段中的学校。

@event = Event.find 4088
@event.schools.joins(:photos).where('photos.event_leg_id' => 28034)

这导致 SQL 将photos_schools表连接两次，一次 toschools和一次 to photos：

SELECT DISTINCT `schools`.* FROM `schools` 
INNER JOIN `photos_schools` `photos_schools_join` ON `photos_schools_join`.`school_id` = `schools`.`id` 
INNER JOIN `photos` `photos_schools_2` ON `photos_schools_2`.`id` = `photos_schools_join`.`photo_id` 
INNER JOIN `photos_schools` ON `schools`.`id` = `photos_schools`.`school_id` 
INNER JOIN `photos` ON `photos_schools`.`photo_id` = `photos`.`id` 
WHERE `photos`.`event_id` = 4088 AND `photos`.`event_leg_id` = 28034

photos_schools -> photos 的第二个 JOIN 是不必要的，下面的查询做同样的事情而且速度更快：

SELECT DISTINCT `schools`.* FROM `schools` 
INNER JOIN `photos_schools` ON `schools`.`id` = `photos_schools`.`school_id` 
INNER JOIN `photos` ON `photos_schools`.`photo_id` = `photos`.`id` 
WHERE `photos`.`event_id` = 4088 AND photos.event_leg_id = 28034;

那么为什么 AR 会创建第二组连接，我怎样才能让它停止呢？

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1 回答 1

1

尝试跳过连接（：照片）我想它应该可以工作

     @event.schools.where('photos.event_leg_id' => 28034)
于 2012-10-27T10:07:21.713 回答