code: def _handle_changed_fields(self, old_data):
          
          for link in self.get_links(is_set=False):
              fld_id = un_camel_id(link['field'])
              if not old_data or old_data.get(fld_id) != self._data[fld_id]:
                  # self is new or linked model changed
                  if self._data[fld_id]:  # exists
                      linked_mdl = getattr(self, link['field'])
                      self._add_back_link(linked_mdl, link)
docs: Looks for changed relation fields between new and old data (before/after save).
          Creates back_link references for updated fields.
  
          Args:
              old_data: Object's data before save.