              # Set the serializer object if it exists
              pk_field_name = self.opts.model._meta.pk.name
              pk_val = native.get(pk_field_name)
              obj = getattr(self.parent.object, field_name)
              if obj and (getattr(obj, pk_field_name) == pk_val):
                  self.object = obj
                  self.delete = native.get('_delete')
  
          obj = self.from_native(native, files)
          if not self._errors:
              self.object = obj
              into[self.source or field_name] = self
          else:
              # Propagate errors up to our parent
              raise ValidationError(self._errors)
 
</s> remove             pk_val = native.get(pk_field_name)
 </s> add  </s> remove             # Propagate errors up to our parent
            raise ValidationError(self._errors)
 </s> add             obj = self.from_native(value, files)
            if not self._errors:
                self.object = obj
                into[self.source or field_name] = self
            else:
                # Propagate errors up to our parent
                raise ValidationError(self._errors) </s> remove             into[self.source or field_name] = self
 </s> add         if value in (None, ''):
            self._delete = True
            into[(self.source or field_name)] = self </s> remove         data = {'id': 3, 'name': u'target-3', 'target_source': {'_delete': True, 'id': 3, 'name': u'target-source-3', 'source': {'id': 3, 'name': u'source-3'}}}
 </s> add         data = {'id': 3, 'name': u'target-3', 'target_source': None} </s> remove         self.delete = False
 </s> add 
              # Set the serializer object if it exists
              pk_field_name = self.opts.model._meta.pk.name
              pk_val = native.get(pk_field_name)
              obj = getattr(self.parent.object, field_name)






              self.object = obj
              into[self.source or field_name] = self
          else:
              # Propagate errors up to our parent
              raise ValidationError(self._errors)
keep keep keep keep replace replace replace replace replace replace keep keep keep keep keep
 Use None to delete nested object as opposed to _delete flag 
rest_framework/serializers.py