title: Django empty field fallback
body: <p>I have a model that holds user address. This model has to have <code>first_name</code> and <code>last_name</code> fields since one would like to set address to a recipient (like his company, etc.). What I'm trying to achieve is:</p>
  
  <ul>
  <li>If the <code>first_name</code>/<code>last_name</code> field in the address is filled - return simply that field</li>
  <li>If the <code>first_name</code>/<code>last_name</code> field in the address is empty - fetch the corrresponding field data from a foreignkey pointing to a proper <code>django.auth.models.User</code></li>
  <li>I'd like this to be treated as normal Django field that would be present in fields lookup</li>
  <li>I don't want to create a method, since it's a refactoring and <code>Address.first_name</code>/<code>last_name</code> are used in various places in the application (also in model forms, etc.), so I need this to me as smooth as possible, or else, I will have to tinker around in a lot of places.</li>
  </ul>