magento rewrite model confusion

Question

I rewrite the class Mage_Sales_Model_Order_Item with these codes in mu config.xml of shopby module

<sales>
    <rewrite>
        <order_item>Amasty_Shopby_Model_Mostordered</order_item>
    </rewrite>
</sales>

and these code is in my mostordered.php in the shopby model folder

class Amasty_Shopby_Model_Mostordered extends Mage_Sales_Model_Order_Item{
    protected  function _beforeSave()
    {
        die(var_dump($this->getData('order_id')));
        return parent::_beforeSave();
    }
}

the code shown in above doesn't work but when i write the die in _beforeSave in item.php in the sales/model/order/item.php it works and it means my rewrite way is wrong in somewhere

protected function _beforeSave()
    {
        die('something');
        parent::_beforeSave();
        if (!$this->getOrderId() && $this->getOrder()) {
            $this->setOrderId($this->getOrder()->getId());
        }
        if ($this->getParentItem()) {
            $this->setParentItemId($this->getParentItem()->getId());
        }
        return $this;
    } 

my only goal is to change the _beforesave method of this php folder where was i mistaking?

update: i found another module that rewrite the same class as i want.so i think because of this my code doesn't work.what is the way to duplicate rewritng?