Changing between forms and subforms

[swee]

I have a combobox that sets a textfield PONumber by using the dmax() function. They are both on the parent form. The parent form and the subform is linked by PONumber. The problem is like this:

I click on the subform and click back on the parent form again. If I choose the same option in the combobox, it will activate the dmax() function again and this time the value of PONumber will be increased by 1. It should display the same value and not activate the function as it is still the current record. Since the PONumber has been incremented by 1, the PONumber in the subform no longer tallies with the main form. This will prompt a violation error message.

Any help greatly appreciated.

Thanks,
Swee