title: Django custom order_by
body: <p>I've got a model eg. Car with a foreign key eg. Owner, which may or may not be blank.
  The Car has a creation_date.</p>
  
  <p>I would like to order these cars by date, but if the car has an owner, the date of birth of the owner must be taken instead of the creation_date of the car.</p>
  
  <p>Is this possible?</p>