protected function getFileName($key, $writable = FALSE) {
    $key = $this->cleanKey($key);

    if ($writable) {
      $this->ensureDirectoryWritable();
    }

    if (!$key) {
      throw new BltException('Could not save data to a file because it is missing an ID');
    }
    return $this->directory . '/' . $key;
  }