Doctrine\ODM\MongoDB\UnitOfWork::getById PHP Method

    public function getById($id, ClassMetadata $class)
    {
        if (!$class->identifier) {
            throw new \InvalidArgumentException(sprintf('Class "%s" does not have an identifier', $class->name));
        }
        $serializedId = serialize($class->getDatabaseIdentifierValue($id));
        return $this->identityMap[$class->name][$serializedId];
    }

 private function loadReferenceManyCollection(PersistentCollection $collection)
 {
     $mapping = $collection->getMapping();
     $cmd = $this->cmd;
     $groupedIds = array();
     foreach ($collection->getMongoData() as $reference) {
         $className = $this->dm->getClassNameFromDiscriminatorValue($mapping, $reference);
         $mongoId = $reference[$cmd . 'id'];
         $id = (string) $mongoId;
         $reference = $this->dm->getReference($className, $id);
         $collection->add($reference);
         if ($reference instanceof Proxy && !$reference->__isInitialized__) {
             if (!isset($groupedIds[$className])) {
                 $groupedIds[$className] = array();
             }
             $groupedIds[$className][] = $mongoId;
         }
     }
     foreach ($groupedIds as $className => $ids) {
         $class = $this->dm->getClassMetadata($className);
         $mongoCollection = $this->dm->getDocumentCollection($className);
         $data = $mongoCollection->find(array('_id' => array($cmd . 'in' => $ids)));
         foreach ($data as $documentData) {
             $document = $this->uow->getById((string) $documentData['_id'], $class->rootDocumentName);
             $data = $this->hydratorFactory->hydrate($document, $documentData);
             $this->uow->setOriginalDocumentData($document, $data);
         }
     }
 }