public static function get($id = null)
    {
        if ($id === null && static::$activeDatabaseConnection === null) {
            throw new MissingDatabaseConnectionException('No databases defined.');
        }

        if ($id === null) {
            return static::$databaseConnections[static::$activeDatabaseConnection];
        } else {
            return static::$databaseConnections[$id];
        }
    }