MongoCollection::createDBRef PHP Method

    public function createDBRef($document_or_id)
    {
        if ($document_or_id instanceof \MongoId) {
            $id = $document_or_id;
        } elseif (is_object($document_or_id)) {
            if (!isset($document_or_id->_id)) {
                return null;
            }
            $id = $document_or_id->_id;
        } elseif (is_array($document_or_id)) {
            if (!isset($document_or_id['_id'])) {
                return null;
            }
            $id = $document_or_id['_id'];
        } else {
            $id = $document_or_id;
        }
        return MongoDBRef::create($this->name, $id);
    }

 /**
  * Do any data prep tasks, such as converting Links to DBRefs.
  *
  * @param array $data
  *   The array of data.
  */
 protected function prepareWriteData(array &$data)
 {
     foreach ($data as $key => $value) {
         if (is_object($value) && $value instanceof Link) {
             $data[$key] = $this->collection->createDBRef($value->getTarget());
         }
     }
 }