In purchase agreement it now has quantity and ordered quantity.

I would like to have a calculated field that shows the remaing quantity: quantity - ordered quantity.

I made a decimal field and put this in calculation:

for record in self:
record[("x_studio_remaining_1")] = record.product_qty - record.qty_ordered

It does nothing and gives an error creating a new order from the purchase agreement. Can anybody point me in the right direction?