public static function create($conn = null, $class = null)
 {
     $q = new ProductQuery($conn);
     $q->addSelect('p.id, p.reference, p.description, p.price')->addSelect($q->quantity_col)->addSelect($q->sold_col)->from("Product p, p.Items i, i.Common inv")->orderBy('p.reference asc')->groupBy('p.id');
     //    echo $q->getSqlQuery();
     return $q;
 }