Doctrine\ODM\MongoDB\PersistentCollection::getMapping PHP Method

    public function getMapping()
    {
        return $this->mapping;
    }

 /**
  * Adds identifiers from a PersistentCollection to $groupedIds.
  *
  * If the relation contains simple references, the mapping is assumed to
  * have a target document class defined. Without that, there is no way to
  * infer the class of the referenced documents.
  *
  * @param PersistentCollection $persistentCollection
  * @param array                $groupedIds
  */
 private function addManyReferences(PersistentCollection $persistentCollection, array &$groupedIds)
 {
     $mapping = $persistentCollection->getMapping();
     if (!empty($mapping['simple'])) {
         $className = $mapping['targetDocument'];
         $class = $this->dm->getClassMetadata($className);
     }
     foreach ($persistentCollection->getMongoData() as $reference) {
         if (!empty($mapping['simple'])) {
             $id = $reference;
         } else {
             $id = $reference['$id'];
             $className = $this->uow->getClassNameForAssociation($mapping, $reference);
             $class = $this->dm->getClassMetadata($className);
         }
         $document = $this->uow->tryGetById($id, $class);
         if (!$document || $document instanceof Proxy && !$document->__isInitialized()) {
             $id = $class->getPHPIdentifierValue($id);
             $groupedIds[$className][serialize($id)] = $id;
         }
     }
 }