created_at = models.DateTimeField(_('creation date'), auto_now_add=True, db_index=True)
changed_at = models.DateTimeField(_('creation date'), auto_now=True, db_index=True)
parent_number = models.IntegerField(_('parent number'), default=0)
created_at = models.DateTimeField(_('creation date'), auto_now_add=True, db_index=True)
changed_at = models.DateTimeField(_('creation date'), auto_now=True, db_index=True)
parent_number = models.IntegerField(_('parent number'), default=0)