docs: def polymorphic_update_timestamp(session, flush_context, instances):
      """This listener ensures an update statement is emited for "entity" table
      to update 'updated_at'.
  
      With joined-table inheritance if the only modified attributes are
      subclass's ones, then no update statement will be emitted.
      """
      for obj in session.dirty:
          if not isinstance(obj, Entity):
              continue
          state = sa.inspect(obj)
          history = state.attrs["updated_at"].history
          if not any((history.added, history.deleted)):
              obj.updated_at = datetime.utcnow()