Load Position by ID or Model

[teone]

Now to use the decrease the quantity of an Item I had to make this cycle:

$positions = Yii::app()->shoppingCart->getPositions();
                foreach($positions as $position) {
                    if ($position->id == $id){
                        $total = $position->getQuantity();
                        Yii::app()->shoppingCart->update($position,$total-1); //1 item with id=1, quantity=2.
                         $price = $position->getSumPrice();
                    } 
                }

Is this right?
If it is a

Yii::app()->shoppingCart->getPosition($book);

to load directly one resource without cycle trought all could be usefull.. Or maybe I just didn't find the right method to do this...

Kindly waiting for a feedback :) Thanks for your great Job!