title: DRF: callable in serializer choicefield "choices"
body: <p>I have an admin serializer with an 'owner' field. I want the current users to populate the choices of this field, but when it comes to running migrations I get a TypeError, suggesting that drf doesnt support callables for the serializer choice field. django supports callables for the model choices field, but obviously the users change on the time, so I want this field to be populated on the serializer instantiation. Can anyone suggest a workable solution here?</p>
  
  <pre><code>def get_available_users():
      return [(u.id, u.username) for u in User.objects.all()]
  
  
  class AdminCreateSerializer(CreateSerializer, AdminSerializer):
      owner = serializers.ChoiceField(choices=get_available_users)
  
  
  &gt;&gt; TypeError: 'function' object is not iterable
  </code></pre>