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

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

Let me pose a typical Rails situation:

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?

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 fancy_hats table.

class FancyHat < ActiveRecord::Base
  belongs_to :person

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