I need to run a formula on each field of a table...

I have a table with some ''class'' and each ''class'' contain some contact. I need to set a constraint that prevent the selection of a contact already in the actual ''class''. It s my first point, how can I do that in the contact field constraint?

For now, I have created some field like : contact_1, contact_2, contact_3,contact_4....and the contact_1 = ID of the first contact of the class, contact_2 = ID of the first contact excluding Contact_1, contact_3 = ID of the first but excluding contact_1 and contact_2...and so on...

but a class can have like 20 contacts, so it s a lot of formula... and my constraint wont work because of the field = null (if I have 3 contact, the contact_4 to contact_20 will be null). So I change the formula to: if (my formula) = null then 0 else (my formula) end

that worked for my test wich included only few fields...but with the 20 fields, when I click on a table line, it start running and wont get back...so I think it s to much formula to evaluate...

Can you help me please? I have look to add a formula in the table updating section, but how can I tell it to check all those fields in all entry?

If you have solution for one of those question, it may help me a lot! :)

thaks!