Allow to customize prefetch behavior in UpdateModelMixin

[maingoh]

I would like to prefetch the related objects of an updated instance. For this purpose I added a prefetch_related_objects in the serializer.save(). However the UpdateModelMixin silently clear it right after.

I suggest moving those two lines in serializer.save() by default

django-rest-framework/rest_framework/mixins.py

Lines 70 to 73 in f222c55

	if getattr(instance, '_prefetched_objects_cache', None):
	# If 'prefetch_related' has been applied to a queryset, we need to
	# forcibly invalidate the prefetch cache on the instance.
	instance._prefetched_objects_cache = {}

and letting the user handling the related objects by himself:

Class MySerializer(serializers.Serializer):
    def save(self):
        instance = super().save()
        prefetch_related_objects([instance], ...)
        return instance

Currently if I want to do this, I need to copy paste the content of UpdateModelMixin.update() in my ModelViewSet and replace this if with my prefetchs.