protected function getRateKey($currencyCode, $date, $rateType, $sourceName)
    {
        return str_replace(
            ['%currency_code%', '%date%', '%rate_type%', '%source_name%'],
            [$currencyCode, $date->format('Y-m-d'), $rateType, $sourceName],
            '%currency_code%_%date%_%rate_type%_%source_name%'
        );
    }