ProductExtension.php

<?php

/**
 * Class ProductExtension
 *
 * @property Product $owner
 */
class ProductExtension extends SiteTreeExtension {

	private static $db = [
		'OriginalPrice' => 'Currency'
	];

	private static $has_one = [
		'Brand' => 'ProductBrand'
	];

	private static $has_many = [
		'ProductImages' => 'ProductImage'
	];


	public function updateCMSFields(FieldList $fields) {


		$fields->dataFieldByName('Image')->setFolderName('Images/Products')->setTitle('Feature Image');
		
		//TRYING: to get the AdditionalImages field added by the other extension
		// Fatal error: Call to a member function setFolderName() on a non-object in /vagrant/httpdocs/mysite/code/Extensions/ProductExtension.php on line 30
		$fields->fieldByName('AdditionalImages')->setFolderName('Images/Products');

		if ($fields->dataFieldByName('BasePrice')) {
			$fields->dataFieldByName('BasePrice')->setTitle('Selling Price')->setDescription('Price the product is currently selling at before modifiers');
		}

		/** @var DropdownField $ddf */
		$fields->addFieldToTab('Root.Main', $ddf = DropdownField::create('BrandID', 'Brand', ProductBrand::get('ProductBrand')->map()->toArray()), 'ParentID');
		$ddf->setHasEmptyDefault(true);

		$fields->addFieldToTab('Root.Pricing',
			TextField::create('OriginalPrice', 'Original price')
			         ->setDescription('Original price to show crossed out if product is on sale')
			         ->setMaxLength(12),
			'CostPrice');

		$fields->removeByName('Variations');

		if ($fields->dataFieldByName('AllowEnquiry')) {
			$fields->dataFieldByName('AllowEnquiry')->setTitle('Allow enquiries on this product (disables purchasing)');
		}

		return $fields;
	}

	/**
	 * Applies the same modifiers over OriginalSellingPrice as Price() does to give us an accurate measure
	 * ensuring we don't end up with the sale price showing as more expensive than the discount price
	 *
	 * @return \ShopCurrency
	 */
	public function getOriginalSellingPrice() {
		$price = $this->owner->OriginalPrice;
		$this->owner->extend("updateSellingPrice", $price);

		//prevent negative values
		$price = $price < 0 ? 0 : $price;

		/** @var ShopCurrency $c */
		$c = Currency::create('OriginalSellingPrice');
		$c->setValue($price);

		return $c;
	}
}