Transaction Notification to prevent changing field in OCRD

Hello guys,

I would like to prevent users from changing the additional identifier field (OCRD.AddID) once it is added. So they can add a new value but can't update or delete it.

I'm unsure how to get the value entered and check it against the existing one. I tried the code below but it didn't behave as I expected. Did anyone have a similar requirement before and how did you do it?

Thanks.

IF

      @object_type = '2' AND @transaction_type IN ('U', 'D')

BEGIN

      IF

            (SELECT AddID FROM ACRD WHERE CardCode=@list_of_cols_val_tab_del AND loginstanc =

            (SELECT MAX(ACRD.LogInstanc) FROM ACRD WHERE CardCode=@list_of_cols_val_tab_del)) IS NULL

            AND

            (SELECT AddID FROM OCRD WHERE CardCode=@list_of_cols_val_tab_del) NOT LIKE

            (SELECT AddID FROM ACRD WHERE CardCode=@list_of_cols_val_tab_del AND loginstanc =

            (SELECT MAX(ACRD.LogInstanc) FROM ACRD WHERE CardCode=@list_of_cols_val_tab_del))

      BEGIN

            SET @error =1

            SET @error_message = 'You cannot change or delete the BPAY ref no.'

      END

END