Stopping calculation over writing data.
I have a 'discount rate' field that is looked up from a 'menu' table as the discount will be the same for all records it is added to so its stored for all to look at the same field.
But how can i stop records that have been completed with the current discount rate eg 2.5% being changed if we change the rate to say 1.5%. The new rate only wants to apply to any new records added after the change.
i.e. if discount_amount is filled in then dont calculate discount
the calculation i have used in defined in the field table for the fields concerned. Do i need to do this via a script ?