protected function mergeRecords(array $records)
    {
        foreach ($records as $key => $record) {
            if (null === $id = array_get($record, static::ID_FIELD)) {
                throw new InternalServerErrorException('No ' .
                    static::ID_FIELD .
                    ' field found in supplied record(s). Cannot merge record(s) for PATCH operation.');
            }

            $rs = $this->getConnection()->getDocument($this->transactionTable, $id);
            $record = array_merge($rs, $record);
            $records[$key] = $record;
        }

        return $records;
    }