title: rails habtm with join model - how to find unlinked records?
body: <p>I have a habtm relationship implemented with a model that represents the join (because the join has attributes):</p>
  
  <pre><code>class Person
    has_many :person_photos
  end
  
  class PersonPhoto
    # has a person and a photo id, and a couple of 
    # other attributes that aren't relevant
  end
  
  class Photo
    has_many :person_photos
  end
  </code></pre>
  
  <p>I'd like to add a method to the Photo class to get a list of people that are <strong>not</strong> in that photo. For the life of me I can't figure it out. Is it easy to do? </p>