轨道4的范围找父母，没有孩子 [英] Rails 4 scope to find parents with no children
问题描述
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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