Doctrine\ODM\MongoDB\MongoDBException::unknownDocumentNamespace PHP Method

    public static function unknownDocumentNamespace($documentNamespaceAlias)
    {
        return new self("Unknown Document namespace alias '{$documentNamespaceAlias}'.");
    }

 /**
  * Resolves a registered namespace alias to the full namespace.
  *
  * @param string $documentNamespaceAlias 
  * @return string
  * @throws MongoDBException
  */
 public function getDocumentNamespace($documentNamespaceAlias)
 {
     if (!isset($this->attributes['documentNamespaces'][$documentNamespaceAlias])) {
         throw MongoDBException::unknownDocumentNamespace($documentNamespaceAlias);
     }
     return trim($this->attributes['documentNamespaces'][$documentNamespaceAlias], '\\');
 }