title: django sort on calculated model field -- can I use .extra?
body: <p>I have the model below.  I want to order by percent_vote.  I know I could calculate the value and save to a new field in the model, but I prefer not to go that way.  Is there some way to use .extra method to do this?</p>
  
  <p>Django 1.6, sqlite3</p>
  
  <pre><code>class HallOfFame(models.Model):
      player = models.ForeignKey(Master)
      year = models.IntegerField(db_index=True)
      voted_by = models.CharField(max_length=30)
      ballots_cast = models.IntegerField()
      votes_needed = models.IntegerField()
      votes_received = models.IntegerField(db_index=True)
      inducted = models.BooleanField(db_index=True)
      category = models.CharField(max_length=30, db_index=True)
      needed_note = models.CharField(max_length=75, blank=True, null=True)
  
  
      def __unicode__(self):
          return "%s %s" % (self.player.name_first, self.player.name_last) 
  
      def percent_vote(self):
          try:
              return self.votes_received/float(self.ballots_cast)
          except ZeroDivisionError:
              return 0
  </code></pre>