gistfile1.php

<?php

$dataColumns = $grid->getConfig()->getComponentByType('GridFieldDataColumns');
$dataColumns->setDisplayFields(array(
	'Title' => array(
		'title' => 'Product',
		'callback' => function($record) {
			return $record->Title . ' - ' . $record->Variant;
		}
	),
	'Price' => 'Price',
	'Order.Surname' => array(
		'title' => 'Customer name',
		'callback' => function($record) {
			$order = $record->Order();
			return $order->FirstName . ' ' . $order->Surname;
		}
	),
	'Order.Paid' => 'Order date'
));