public function json_encode($code = true)
 {
     // 2 basic properties of the sold product must have
     $json = parent::json_encode(false);
     $json['import_price'] = $this->import_price;
     $json['object_type'] = $this->object_type;
     // code = true, return json encode, else just return object data encode as an array
     if ($code) {
         return json_encode($json);
     } else {
         return $json;
     }
 }