Define unique record

[new comer]

Hi, I have two related DBs A and B, with 1:m relationship. I created a auto-enter serial # in A as primary key, and set up relationship using this field.

Now I need to provent user creating duplicate record in B. In B, a record can be identified by 3 fields (serial#, OTA code, and date).

What I did now is create a text field, auto-enter calculate value(which hold the 3 fields), and I check the unique radio box in validation. But it doesn't work.

Can someone please tell me how to do this? Thank in advance!