protected function findOneByKeyFromCache($key, $value)
    {
        if ($key === '') {
            throw new \InvalidArgumentException('$key must not be empty.', 1416847364);
        }
        if (!isset($this->cacheByKey[$key])) {
            throw new \InvalidArgumentException('"' . $key . '" is not a valid key for this mapper.', 1331319882);
        }
        if ($value === '') {
            throw new \InvalidArgumentException('$value must not be empty.', 1331319892);
        }

        if (!isset($this->cacheByKey[$key][$value])) {
            throw new \Tx_Oelib_Exception_NotFound();
        }

        return $this->cacheByKey[$key][$value];
    }