Related Products

<?php
class ProductPage extends Page {
	private static $many_many = array(
		'RelatedProducts' => 'Product'
	);

	public function getCMSFields() {
		$fields = parent::getCMSFields();

		$products = ProductPage::get()->exclude(array('ID' => $this->owner->ID));
		$gridField = new GridField('products', 'Related Products', $products);
		$gridField->getConfig()->addComponent(new GridFieldManyRelationHandler($this, 'RelatedProducts'));

		$fields->addFieldToTab('Root.RelatedProducts', $gridfield);
	}
}