Update query using dynamic data in zend?

Question

I am using zend, I have the following code like

class Admin_Model_DbTable_Inv extends Zend_Db_Table_Abstract
{
    protected $_name = 'test';
    public function updateproductstock($qty,$pid)
    {
        $data = array(
            'stock' => 'stock - 2'
        );
        $this->_db->update($this->_name, $data, product = '.$pid.');
    } 
}

Here,I want to subtract my qty from stock.For example 2 will be qty. qty will be dynamic data .How can i use zend update here. kindly help.

Updated my post

Answer 1

 class Admin_Model_DbTable_Inventory extends Zend_Db_Table_Abstract
 {
     protected $_name = 'test';

     public function updateproductstock($qty,$pid)
     {
         $data = array(
             'stock' => new Zend_Db_Expr('stock - '.$qty);
         );
         $this->_db->update($this->_name, $data, product = '.$pid.');
     }
 }