public function process()
    {
        $operands = $this->getOperands();

        if ($operands->exclusivelyRecord() === false) {
            $msg = "The FieldValue operator only accepts operands with a cardinality of record.";
            throw new OperatorProcessingException($msg, $this, OperatorProcessingException::WRONG_CARDINALITY);
        }

        $fieldIdentifier = $this->getExpression()->getFieldIdentifier();

        return $operands[0][$fieldIdentifier];
    }