title: Django REST Framework Partial update and validation
body: <p>I want to perform a partial update on my serializer. The problem is that I have a some validation at object level. So the <code>is_valid()</code> call always fails and I can't save the the updated serializer.
  Can I somehow prevent the object level validation on partial updates?
  Here a code sample:</p>
  
  <pre><code>class ModelSerializer(serializers.ModelSerializer)
      class Meta:
          model = A
          fields = ('field_b','field_b')
  
      def validate(self,attrs):
           if attrs.get('field_a') &lt;= attrs.get('field_b'):
               raise serializers.ValidationError('Error')
  </code></pre>
  
  <p>And in my viewset the partial update method:</p>
  
  <pre><code>class ModelViewSet(viewsets.ModelViewSet):
      def partial_update(self, request, *args, **kwargs):
          instance = self.get_object()
          serializer = self.serialize(instance, data=request.data, partial=True)
      serializer.is_valid(raise_exception=True)
      new_instance = serializer.save()
      return Response(serializer.data)
  </code></pre>
  
  <p>The problem ist that I cannot update 'field_a' without 'field_b'.
  Thanks for your help!  </p>