public static function fetchByClientId( $clientId )
    {
        $session = ezcPersistentSessionInstance::get();

        $q = $session->createFindQuery( __CLASS__ );
        $q->where( $q->expr->eq( 'client_id', $q->bindValue( $clientId ) ) );
        $results = $session->find( $q, __CLASS__ );
        if ( count( $results ) != 1 )
            return false;
        else
            return array_shift( $results );
    }