devroom.io/content/posts/2009/2009-06-07-has_one-find-all-that-have-no-associated-object.md

+++
date = "2009-06-07"
title = "has_one - find all that have no associated object"
tags = ["General", "Ruby", "Rails", "has_one", "has_many", "belongs_to"]
slug = "has_one-find-all-that-have-no-associated-object"
+++

Let me pose a typical Rails situation:

```ruby
class Person < ActiveRecord::Base
  has_one :fancy_hat
end

class FancyHat < ActiveRecord::Base
  belongs_to :person
end
```

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

```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
```

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.

```ruby
class FancyHat < ActiveRecord::Base
  belongs_to :person

  named_scope :wearerless, :conditions => { :person_id => nil }
end
```