/**
* Add the compound profile_id/event_id index to our cache keys
* since the DB_DataObject stuff doesn't understand compound keys
* except for the primary.
*
* @return array
*/
function _allCacheKeys()
{
$keys = parent::_allCacheKeys();
$keys[] = self::multicacheKey('RSVP', array('profile_id' => $this->profile_id, 'event_id' => $this->event_id));
return $keys;
}