title: Finding records with no associated records in rails 3
body: <pre><code>class Person &lt; ActiveRecord::Base
    has_many :pets
  
    scope :with_dog, join(:pets).where("pets.type = 'Dog'")
    scope :without_pets ???????????????????????????????????
  end
  
  class Pet &lt; ActiveRecord::Base
    belongs_to :people
  end
  </code></pre>
  
  <p>I'd like to add a scope to the Person model that returns people who have no pets.  Any ideas?  I feel like this is obvious, but it's escaping me at the moment.</p>