Cakephp sql Update: add up old field value with value from form

So I am relatively new to the MVC world. What I am doing at the moment is rewriting a php project in CakePHP2.0. Currently I am working on orders, more specifically the delivery of products/stock.

This is the previous sql code for a delivery of products:

$sqlSelect =("SELECT current_stock FROM stocks WHERE id IN (");
$sql = $con->prepare("UPDATE orders SET order_status=1 WHERE order_number=:orderNumber");
$sql2 = $con->prepare("UPDATE stocks SET current_stock= :stock + :delivered  WHERE id =     :stockID");

try {

$con->beginTransaction();

for($count=0; $count<=$_POST['count']; $count++) {

$idQuery [] ="?,";
$idArray []=($_POST['stock_id' .$count]);

}

$sqlSelect .= implode($idQuery);
$sqlSelect =trim($sqlSelect, ",") . ")"; // removes last comma from string, then closes  the bracket //
$stmt =$con->prepare($sqlSelect);
$stmt->execute($idArray);

while ($row=$stmt->fetch()) {

$stock = ($row['current_stock']);

$sql2->bindParam(':stock', $stock);
}

for($count=0; $count<=$_POST['count']; $count++) {

$sql2->bindParam(':delivered', $_POST['delivery_amount' .$count]);
$sql2->bindParam(':stockID', $_POST['stock_id' .$count]);
$sql2->execute();

}

$sql->bindParam(':orderNumber', $_POST['order_num'] );


$sql->execute();
$con->commit();
$con=null;

 echo "<script language=javascript>
    alert('Delivery confirmed. Your stocks have been adjusted.')
location.replace('placeorder2.php')
</script>";


} catch (Exeception $e) {

$con->rollback();
echo "Update Failed: " . $e->getMessage();

}
?>

What I would like to know is what is the easiest way to perform this the 'CakePHP' way?

My Associations are as follows: Product hasOne Stock, Product hasMany Order.