protected function loadExchangeRates($targetCurrency)
    {
        if (!isset($this->exchangeRates[$targetCurrency])) {
            $currencyRates = $this->currencyRateRepository->findBy(['currencyTo' => $targetCurrency]);
            if (count($currencyRates) === 0) {
                throw new MissingCurrencyRatesException($targetCurrency);
            }
            foreach ($currencyRates as $rate) {
                $this->setExchangeRate($rate, $targetCurrency);
            }
        }
    }