Blocking the change of Supplier Invoice Number on update using SP

Question

Hi Mentors,
Just wondering if blocking the change of Supplier Invoice No. (OPCH.NumAtCard) is possible using SP_TransactionNotification?
I've been trying to make it work with the following code:
--Prevent Duplicate Supplier Ref. No. in AP Invoice
if @object_type = '18' and (@transaction_type in ('A' ,'U'))
begin
declare @RefNo as varchar (100)
declare @CardCode as varchar (100)
     if (@object_type = '18')
          begin
             select @RefNo = NumAtCard, @CardCode = CardCode from OPCH T0 where docentry = @list_of_cols_Val_tab_del
             begin
               if (@RefNo is not null)
                 begin
                   if 1!= (select count (docentry) from OPCH  where (NumatCard = @RefNo) and (cardcode = @CardCode))
                     begin
                        select @error = 10
                        select @error_message = 'Duplicate Supplier Reference Number!'
                     End
                   End
               End
          End
End
--Prevent Supplier Ref. No. in AP Invoice from being changed during update mode.
if @object_type = '18' and (@transaction_type = 'U')
begin
declare @DocEntry as int
     if (@object_type = '18')
          begin
             select @RefNo = NumAtCard, @CardCode = CardCode , @DocEntry = DocEntry from OPCH T0 where DocEntry = @list_of_cols_Val_tab_del
             begin  
                   if (@RefNo != (select NumAtCard from OPCH where (CardCode = @CardCode) and (DocEntry = @DocEntry)))
                     begin
                        select @error = 10
                        select @error_message = 'Changing of Supplier Reference Number is not allowed!'
                     End
                   End
          End
End
I'm not sure if this is really possible,and that I could be just beating the wind by trying.
Thanks in advance!
Sean

Answer 1

To avoid modifiing the vandor reference use this:
IF @Object_type = N'18' and @transaction_type = N'U'
BEGIN     ---
declare @li int
declare @pref nvarchar(100)
set @li=
(select max(t.LogInstanc)   from ADOC t
   where t.ObjType=18 and t.DocEntry=@list_of_cols_val_tab_del)
set @pref=
(select isnull(t.NumAtCard,'')    from ADOC t
    where t.ObjType=18 and t.DocEntry=@list_of_cols_val_tab_del
      and t.LogInstanc=@li-1)
If @pref !='' and @pref !=
(select isnull(t.NumAtCard,'')   from OPCH t
   where t.DocEntry=@list_of_cols_val_tab_del)
Select @error = 10, @error_message = N'Changing of Supplier Reference Number is not allowed!'
END     
--- To avoid using duplicated reference try this:
if @object_type = '18' and @transaction_type in (N'A', N'U') -- AP invoice
BEGIN          ----
declare @Invoice AS VARCHAR(15)
declare @Card AS VarChar (20)
SELECT @invoice = NumatCard, @card = CardCode FROM dbo.OPCH
     WHERE DocEntry = @list_of_cols_val_tab_del
If exists (SELECT T0.cardcode, T0.NumatCard FROM OPCH T0 where T0.NumatCard = @invoice
           and T0.CardCode=@card and T0.DocEntry!=@list_of_cols_val_tab_del)
Select error = 10, @error_message = N'Duplicate Supplier Reference Number!'
END          ----