private function prepareKey($storageName, $key)
    {
        if (is_array($key)) {
            $keyValue = reset($key);
            $keyName  = key($key);
        } else {
            $keyValue = $key;
            $keyName  = $this->getKeyNameForTable($storageName);
        }

        return $this->marshaler->marshalItem([$keyName => $keyValue]);
    }