def _handle_changed_fields(self, old_data):
for link in self.get_links(is_set=False):<EOL><INDENT>fld_id = un_camel_id(link['<STR_LIT>'])<EOL>if not old_data or old_data.get(fld_id) != self._data[fld_id]:<EOL><INDENT>if self._data[fld_id]:  <EOL><INDENT>linked_mdl = getattr(self, link['<STR_LIT>'])<EOL>self._add_back_link(linked_mdl, link)<EOL><DEDENT><DEDENT><DEDENT>
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.