---
title: "has_one - find all that have no associated object"
kind: article
slug: has_one-find-all-that-have-no-associated-object
created_at: 2009-06-07
tags:
  - General
  - Ruby
  - Rails
  - has_one
  - has_many
  - belongs_to
---

Let me pose a typical Rails situation:

<pre lang="ruby">class Person < ActiveRecord::Base
  has_one :fancy_hat
end

class FancyHat < ActiveRecord::Base
  belongs_to :person
end</pre>

Now, how can you get all the people that don't have a fancy hat?

<pre lang="ruby">class Person < ActiveRecord::Base
  has_one :fancy_hat

  named_scope :hatless, :joins => 'LEFT JOIN fancy_hats ON fancy_hats.person_id = people.id', :conditions => 'fancy_hats.person_id IS NULL'
end</pre>

Now you can find all the hatless people you want.

FYI: Finding fancy hats that have no one to wear them is a lot easier, because the foreign key is stored in the <code>fancy_hats</code> table.

<pre lang="ruby">class FancyHat < ActiveRecord::Base
  belongs_to :person

  named_scope :wearerless, :conditions => { :person_id => nil }
end</pre>