code: def default_format(self):
          
          user = self.user
          if user.first_name is not None:
              return self.full_name
          elif user.username is not None:
              return user.username
          else:
              return str(user.id)
docs: Returns full name (first and last) if name is available.
          If not, returns username if available.
          If not available too, returns the user id as a string.