          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)
  
      def get_default_fields(self):
          """
          Return all the fields that should be serialized for the model.
          """
 
</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             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:
 </s> add  </s> remove             pk_val = native.get(pk_field_name)
 </s> add  </s> remove             native = data[field_name]
 </s> add             value = data[field_name]
            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)
          if not self._errors:
              self.object = obj
              into[self.source or field_name] = self
          else:
            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)
            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)
  
      def get_default_fields(self):
          """
          Return all the fields that should be serialized for the model.
          """
keep keep keep keep replace replace keep keep keep keep keep
 Use None to delete nested object as opposed to _delete flag 
rest_framework/serializers.py