CategoryPage.php

<?php
class CategoryPage extends Page{
	private static $has_one = array(
		'MainCategoryPage' => 'MainCategoryPage'
	);

	public function getCMSFields() {
		$fields = parent::getCMSFields();
		//$fields->removeByName(array());

		$mains = MainCategoryPage::get()->sort('Title DESC')->map('ID', 'Title');
		$fields->insertBefore(DropdownField::create('MainCategoryPageID', 'Type of product', $mains), 'Title');

		return $fields;
	}
}

class CategoryPage_Controller extends Page_Controller {

}

Page.php

<?php
class Page_Controller extends ContentController {
public function addTagFilters( $filter ){ //Example URL domain.td?healthy=1&cheap=1
		$request = $this->getRequest();
		$params = $request->getVars();
		if(count($request->getVars()) > 1 ){
			unset($params['url']); //Remove url from the request, so we only have the get variables
			$filterable_params = array();
			foreach($params as $key => $val){
				array_push($filterable_params, $key);
			}
			$filter['Tags.Name'] = $filterable_params;
		}
		return $filter;
	}
	public function RelatedProducts(){
		$zip = Session::get('ZipSort');

		$filter = array('Quantity:GreaterThan' => 0);

		if($zip){
			$filter['Zip'] = $zip;
		}

		$prods = ProductPage::get();

		if($this instanceof MainCategoryPage_Controller || $this->ClassName == 'CategoryPage'){
			$filter = $this->addTagFilters($filter);
			$prods = $prods->filter($filter)->sort('Created DESC');
		}

		return $prods;
	}
}

Tag.php

<?php
class Tag extends DataObject{
	private static $db = array(
		'Name' => 'Varchar',
	);
	private static $has_one = array(
		'MainCategoryPage' => 'MainCategoryPage'
	);
	private static $summary_fields = array(
		'Name' => 'Name',
		'MainCategoryPage.Title' => 'MainCategory'
	);

	public function getCMSFields() {
		$fields = parent::getCMSFields();
		//$fields->removeByName(array());
		$fields->addFieldsToTab('Root.Main', TextField::create('Name'));
		$mains = MainCategoryPage::get()->sort('Title DESC')->map('ID', 'Title');
		$fields->addFieldsToTab('Root.Main', DropdownField::create('MainCategoryPageID', 'Type of product', $mains));

		return $fields;
	}
}