  
          for field_name, field in self.fields.items():
              field.initialize(parent=self, field_name=field_name)
              try:
                  field.field_from_native(data, files, field_name, reverted_data)
              except ValidationError as err:
                  if hasattr(err, 'message_dict'):
 
</s> remove     target = models.OneToOneField(OneToOneTarget, related_name='target_source')
 </s> add     target = models.OneToOneField(OneToOneTarget, null=True, blank=True,
                                  related_name='target_source')
            if isinstance(field, ModelSerializer) and self.object:
                # Set the serializer object if it exists
                pk_field_name = field.opts.model._meta.pk.name
                obj = getattr(self.object, field_name)
                nested_data = data.get(field_name)
                pk_val = nested_data.get(pk_field_name) if nested_data else None
                if obj and (getattr(obj, pk_field_name) == pk_val):
                    field.object = obj
                    field.delete = nested_data.get('_delete')
  
          for field_name, field in self.fields.items():
              field.initialize(parent=self, field_name=field_name)
            if isinstance(field, ModelSerializer) and self.object:
                # Set the serializer object if it exists
                pk_field_name = field.opts.model._meta.pk.name
                obj = getattr(self.object, field_name)
                nested_data = data.get(field_name)
                pk_val = nested_data.get(pk_field_name) if nested_data else None
                if obj and (getattr(obj, pk_field_name) == pk_val):
                    field.object = obj
                    field.delete = nested_data.get('_delete')
              try:
                  field.field_from_native(data, files, field_name, reverted_data)
              except ValidationError as err:
                  if hasattr(err, 'message_dict'):
keep keep add keep keep keep keep
 Add one-to-one nested update and delete functionality 
rest_framework/serializers.py