magento 1.9不会在sales_order_save_after上保存数量值

Question

When the event sales_order_save_after is triggered the new quantity value is not inserted, what is the problem? How to update quantity when this event is triggered? The eventi is triggered twice.

    public function save_product_frontend($observer = null) { 
       $event = $observer->getEvent();
       $order = $event->getOrder();
       $items = $order->getAllItems();
       foreach ($items as $item) {
          $product = Mage::getModel("catalog/product")->load( $item->getProductId() );
          $stockitem = Mage::getModel('cataloginventory/stock_item')->loadByProduct( $product );
          $stockitem->setQty( 100 );
          $stockitem->save();
      }
  }