Rails 查找与零相关的 has_many 记录

这似乎相当简单，但我不能让它出现在谷歌上。

如果有:

class City < ActiveRecord::Base
has_many :photos
end


class Photo < ActiveRecord::Base
belongs_to :city
end

我想找到所有没有照片的城市。我想能够调用类似..。

City.where( photos.empty? )

... 但是这并不存在。那么，你怎么做这种查询呢？

更新: 现在已经找到了原始问题的答案，我很好奇，你如何构造相反的结构？

IE: 如果我想创建这些作用域:

scope :without_photos, includes(:photos).where( :photos => {:city_id=>nil} )
scope :with_photos, ???
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最佳答案

Bah, found it here: https://stackoverflow.com/a/5570221/417872

City.includes(:photos).where(photos: { city_id: nil })
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When trying to find records with no matching records from the joined table, you need to use a LEFT OUTER JOIN

scope :with_photos, joins('LEFT OUTER JOIN photos ON cities.id = photos.city_id').group('cities.id').having('count(photos.id) > 0')
scope :without_photos, joins('LEFT OUTER JOIN photos ON cities.id = photos.city_id').group('cities.id').having('count(photos.id) = 0')
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I used a join to get all the ones with photos:

scope :with_photos, -> { joins(:photos).distinct }

Easier to write and understand, for that particular case. I'm not sure what the efficiency is of doing a join vs doing an includes, though

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In Rails versions >= 5, to find all cities that have no photos, you can use left_outer_joins:

City.left_outer_joins(:photos).where(photos: {id: nil})

which will result in SQL like:

SELECT cities.*
FROM cities LEFT OUTER JOIN photos ON photos.city_id = city.id
WHERE photos.id IS NULL

Using includes:

City.includes(:photos).where(photos: {id: nil})

will have the same result, but will result in much uglier SQL like:

SELECT cities.id AS t0_r0, cities.attr1 AS t0_r1, cities.attr2 AS t0_r2, cities.created_at AS t0_r3, cities.updated_at AS t0_r4, photos.id AS t1_r0, photos.city_id AS t1_r1, photos.attr1 AS t1_r2, photos.attr2 AS t1_r3, photos.created_at AS t1_r4, photos.updated_at AS t1_r5
FROM cities LEFT OUTER JOIN photos ON photos.city_id = cities.id
WHERE photos.id IS NULL
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I don't believe the accepted answer gives you exactly what you're looking for, as you want to do a LEFT OUTER JOIN and that answer will give you a INNER JOIN. At least in Rails 5 you can use:

scope :without_photos, left_joins(:photos).where( photos: {id: nil} )

or you can use merge in cases where namespacing will make the where clause cumbersome:

scope :without_photos, left_joins(:photos).merge( Photos.where(id: nil) )
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If you are not running Rails 5+ and performance is a must-have, avoid useless ActiveRecord creation and get just what you need:

City.where("NOT EXISTS(SELECT 1 FROM photos WHERE photos.city_id = cities.id LIMIT 1)")

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