title: Django Admin Form Limit Dropdown Options
body: <p>So in the Django admin I have an object change form like so:</p>
  
  <pre><code>class SurveyChoiceField(forms.ModelChoiceField):
      def label_from_instance(self, obj):
          return u'{0} - {1}'.format(obj.id, obj.name)
  
  
  class BlahAdminForm(forms.ModelForm):
      def __init__(self, *args, **kwargs):
          super(BlahAdminForm, self).__init__(*args, **kwargs)
          surveys = Survey.objects.filter(deleted=False).order_by('-id')
          self.fields['survey'] = SurveyChoiceField(queryset=surveys)
  
  
  class BlahAdmin(admin.ModelAdmin):
      form = BlahAdminForm
  </code></pre>
  
  <p>and I would like to limit the dropdown to surveys of a certain type based on the blah. Something like</p>
  
  <pre><code>blah_id = self.blah.id
  blah_survey_type = Blah.objects.filter(id=blah_id).get('survey_type')
  surveys = Survey.objects.filter(deleted=False, type=blah_survey_type).order_by('-id')
  </code></pre>
  
  <p>but I'm not sure how to get the <code>id</code> of the Blah in the <code>BlahAdminForm</code> class.</p>