thank you for your suggestion anthony, i've already tested it, both db(
db.product.id==key).update(quantity=db.product.quantity - value)
and db(db.product.id==key).update(quantity=db.product(key).quantity -
value) is work well during update the quantity in product table.
db(db.product.id==key).update(quantity=db.product.quantity -
>> value)
>
>
> Here you probably want:
>
> db(db.product.id==key).update(quantity=db.product(key).quantity -value
> )
>
> db.product.quantity is a Field object, but you want to get the value of
> that field for the particular record, which is db.product(key).quantity.
>
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