public function getObjectGraph($className, $graph, $criteria= null, $cacheFlag= true) {
        $object= null;
        if ($collection= $this->getCollectionGraph($className, $graph, $criteria, $cacheFlag)) {
            if (!count($collection) === 1) {
                $this->log(xPDO::LOG_LEVEL_WARN, 'getObjectGraph criteria returned more than one instance.');
            }
            $object= reset($collection);
        }
        return $object;
    }