我发现一个答案说了一些有用的为例子寻找父母 N 的孩子，但同样不是找父母没有子女可用（presumably自加盟排除它们）。

I found one answer that had some usable having examples for finding parents with n children, but the same is not usable for finding parents with no children (presumably since the join excludes them).

scope :with_children, joins(:children).group("child_join_table.parent_id").having("count(child_join_table.parent_id) > 0")

任何人都可以点我朝着正确的方向？

Can anyone point me in the right direction?

推荐答案

这应该做你想要的工作：

This should do the job you want:

scope :without_children, includes(:children).where(:children => { :id => nil })

这里最大的区别是加入成为包括：一个包括加载所有关系，如果他们存在，连接只会加载关联对象，而忽略对象没有关系。

The big difference here is the joins becoming a includes: an include loads all the relations, if they exists, the join will load only the associated objects and ignore the object without a relation.

其实，范围：with_children，加入（：儿童）应该是刚够至少有1名儿童返回父。试试吧！

In fact, scope :with_children, joins(:children) should be just enough to return the Parent with at least 1 child. Try it out!

由于@MauroDias指出，如果这是你的父母和孩子，这code以上将无法正常工作的一种自我指涉的关系

As @MauroDias pointed out, if it is a self-referential relationship between your parent and children, this code above won't work.

随着研究一点点，我发现了如何做到这一点：

With a little bit of research, I found out how to do it:

考虑这个模型：

class Item < ActiveRecord::Base
  has_many :children, :class_name => 'Item', :foreign_key => 'parent_id'

如何返回，没有子女的所有项目：

How to return all items with no child(ren):

Item.includes(:children).where(children_items: { id: nil })

我是怎么找到 children_items 表？

Item.joins（：儿童）生成的SQL语句：

SELECT "items".* 
FROM "items" 
 INNER JOIN "items" "children_items" 
 ON "children_items"."parent_id" = "items"."id"

所以我猜Rails使用的表需要在自我指涉的情况下，JOIN的时候。

So I guessed that Rails uses a table when in need of a JOIN in a self-referential case.

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