continuing the Django polls tutorial, I want an entry (I called it numChoices) in the Poll model that automatically updates with the number of Choices associated with that Poll. How can I do that?

```
class Poll(models.Model):
    question = models.CharField(max_length=200)
    pub_date = models.DateTimeField('date published')
    numChoices = models.IntegerField()
    def __unicode__(self):
        return self.question
    def was_published_recently(self):
        now=timezone.now()
        return now-datetime.timedelta(days=1) <= self.pub_date < now
    was_published_recently.admin_order_field = 'pub_date'
    was_published_recently.boolean = True
    was_published_recently.short_description = 'Published recently?'

class Choice(models.Model):
    poll = models.ForeignKey(Poll)
    choice_text = models.CharField(max_length=200)
    votes = models.IntegerField(default=0)
    def __unicode__(self):
        return self.choice_text

```

Clarification, the ideal use case would be such that:

If i just want to list all the polls with the number of choices associated with each poll, it won't have to query the Choice table.

But every time i add a choice, the poll entry associated with that choice will update it's numChoices count