onBeforeWrite.php

public function onBeforeWrite(){

		if(!$this->CategoryID){
			$default = ProductCategory::get()->filter(array('Code' => 'DEFAULT'))->first();
			$this->CategoryID = $default->ID;
		}

		//update many_many lists when multi-group is on
		if(SiteConfig::current_site_config()->MultiGroup){
			$holders = $this->getManyManyComponents('ProductHolders');
			$product = ProductPage::get()->byID($this->ID);
			if (isset($product->ParentID)) {
				$origParent = $product->ParentID;
			} else {
				$origParent = null;
			}
			$currentParent = $this->ParentID;
			if($origParent!=$currentParent){
				if($holders->find('ID', $origParent)){
					$holders->removeByID($origParent);
				}

			}
			$holders->add($currentParent);
		}

		$title = ltrim($this->Title);
		$title = rtrim($title);
		$this->Title = $title;

		parent::onBeforeWrite();

	}