title: Django REST Framework: default fields in browseable API form
body: <p>I have a model:</p>
  
  <pre><code>class XCall(models.Model):
      created_on = models.DateTimeField(auto_now_add=True)
      send_on = models.DateTimeField(default=datetime.now)
      recipient = models.ForeignKey(User)
      text = models.CharField(max_length=4096)
      backup_calls = models.IntegerField(blank=True, null=True)
  </code></pre>
  
  <p>And a serializer for that model:</p>
  
  <pre><code>class CallSerializer(serializers.HyperlinkedModelSerializer):
      url = serializers.HyperlinkedIdentityField(
          view_name='call-detail',
      )
      # some validation and custom field definitions
      ...
  
      class Meta:
          model = XCall
          fields = ('url', 'id', 'text', 'recipient', 'send_on', 'backup_calls', 'status')
          lookup_field= 'pk'
  </code></pre>
  
  <p>And here's the list view:</p>
  
  <pre><code>class CallList(generics.ListCreateAPIView):
      serializer_class = CallSerializer
      permission_classes = (permissions.IsAuthenticatedOrReadOnly, IsOwnerOrSuperuser,)
  
      def pre_save(self, obj):
          auth_user = self.request.user
          obj.auth_user = auth_user
  
      def get_queryset(self):
          """
          This view should return a list of all the calls
          for the currently authenticated user.
          """
          auth = self.request.user
          if isinstance(auth, AnonymousUser):
              return []
          elif auth.is_superuser:
              return XCall.objects.all()
          else:
              return XCall.objects.filter(auth_user=auth)
  </code></pre>
  
  <p>In <code>CallList</code>'s browseable API, I see the following in the POST form at the bottom:
  <img src="https://i.stack.imgur.com/etTNR.png" alt="enter image description here"></p>
  
  <p>My question is: why is there no default value set for <code>send_on</code>, and there is one for <code>backup_calls</code>? I assumed that the form would follow the <code>XCall</code> model specification and use <code>datetime.now()</code> for defaulting the former, and leave <code>backup_calls</code> blank (since it's nullable). How can I get the form to follow the model specifications?</p>