title: limiting the queryset for ManyToMany MultipleSelect in Django admin
body: <p>I have a model that looks like this:</p>
  
  <pre><code>class Event(models.Model):
      event_dates = ManyToManyField("EventDate")
      #...
  
  class EventDate(models.Model):
      event_date = DateField()
      #...
  </code></pre>
  
  <p>However, in the django admin <code>MultipleSelect</code> form field that gets show for event_dates in the <code>EventAdmin</code>, I'd like to limit the queryset to event_dates that are not in the past. </p>
  
  <p>The queryset would be something like:</p>
  
  <pre><code>event_date_queryset = EventDate.objects.filter(event_date__gte = datetime.date.today()) 
  </code></pre>
  
  <p>But where can I set this queryset so that only non-past dates show up in the field? </p>
  
  <p>(I don't currently have a custom form for the <code>EventAdmin</code> but would be happy to add one.) </p>