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!
Rails 5
See @Anson's answer below
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.
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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