Rails 4 查找没有孩子的父母

问题描述

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")

Can anyone point me in the right direction?

解决方案

Update Rails 6.1

With the new Rails version this becomes simple, as described here:

.where.missing(:children)

For older versions see below.

Rails 3 & 4

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

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.

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

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Rails 5

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Gem activerecord_where_assoc

The activerecord_where_assoc gem can do this for Rails 4.1 up to 6.0.

scope :without_children, where_assoc_not_exists(:children)

Self-referencing relation are handled seemlessly.

This also avoids issues such as joins making the query return multiple rows for a single record.

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

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

Consider this model:

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

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

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

How did I find that children_items table?

Item.joins(:children) generates the following SQL:

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

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

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