Limited number of entries in a subform with data validation

[jaworski_m]

Hello,

I have a form where currencies are chosen for a Purchase Order (PO).
More then one currency can be defined for a single PO, but 3 is max.

Currency groups:
(1) EUR, USD
(2) (other currencies: GBP, CHF, other1, other2, ..., other n)

Currencies from group (1) can ALWAYS be selected but only ONE from group (2) can be chosen.

For example (max 3, min 1):
CORRECT choices:
- EUR, USD, GBP
- EUR, GBP
- GBP
- EUR

INCORRECT choices:
- EUR, GBP, CHF (either GBP or CHF can be selected)


- GBP, CHF

How can such data validation be achieved? Table level? Form's event?

Thank you for hints.

[NTC]

well there is more than 1 way to approach this....I think I might put them in a table - and the table is the record source for your drop down selector (combo box); in the table I would add this second numeric value:
EUR 0
USD 0
GBP 1
CHF 1
that numeric value doesn't have to be seen by the user; but then you can trigger simple math in the AfterUpdate event of the selection control using this second number - and if =2 then you can pop up a message or otherwise stop the person from what they are doing...