title: Creating a dynamic choice field
body: <p>I'm having some trouble trying to understand how to create a dynamic choice field in django. I have a model set up something like:</p>
  
  <pre><code>class rider(models.Model):
       user = models.ForeignKey(User)
       waypoint = models.ManyToManyField(Waypoint)
  
  class Waypoint(models.Model):
       lat = models.FloatField()
       lng = models.FloatField()
  </code></pre>
  
  <p>What I'm trying to do is create a choice Field whos values are the waypoints associated with that rider (which would be the person logged in).</p>
  
  <p>Currently I'm overriding init in my forms like so:</p>
  
  <pre><code>class waypointForm(forms.Form):
       def __init__(self, *args, **kwargs):
            super(joinTripForm, self).__init__(*args, **kwargs)
            self.fields['waypoints'] = forms.ChoiceField(choices=[ (o.id, str(o)) for o in Waypoint.objects.all()])
  </code></pre>
  
  <p>But all that does is list all the waypoints, they're not associated with any particular rider. Any ideas? Thanks.</p>