SQL trigger trouble with TRY--CATCH block for INSERT command - sql

Question

Ok, so I want to create a trigger that fires when someone tries to insert into the table "products" and checks for a valid foreign key. For right now (This is NOT the end design in the least, but I'm using it for testing), I want the trigger to check that the inserted line references a valid ID in the Manufacturer table, and if no such row exists in the Manufacturer table, insert one with the proper ID and some general information for the remaining fields. My current code is as follows:
create trigger checkman
on dbo.products
instead of insert
as
declare
#manid char(5),
#manName varchar(50),
#transactionName varchar(20) = 'transaction1'
Begin
select #manid=Man_ID from Inserted
begin try
/*begin tran #transactionName*/
Insert into Manufacturers (Man_ID, Man_Name, Man_Description) VALUES #manid, 'Unknown Name', 'This is an unknown manufacturer');
insert into dbo.products select * from inserted;
end try
begin catch
/*rollback tran #transactionName;*/
insert into dbo.products select * from inserted;
end catch
End
The problem is that whenever I run my insert with a Manufacturer ID that already exists, I get this error:
(0 row(s) affected)
Msg 3930, Level 16, State 1, Procedure checkman, Line 20
The current transaction cannot be committed and cannot support operations that write to the log file. Roll back the transaction.
The statement has been terminated.
I get the same error when I put the "Insert into Manufacturers" line in the catch block, only this time it appears when I try to insert with an ID that doesn't yet exist.

Answer 1

Don't handle this in a try/catch block, it isn't necessary. Also you need to allow for the fact that inserted can contain multiple rows. You can do this using a NOT EXISTS query:
CREATE TRIGGER CheckMan
ON dbo.products
INSTEAD OF INSERT
AS
INSERT Manufacturers (Man_ID, Man_Name, Man_Description)
SELECT DISTINCT Man_ID, 'Unknown Name', 'This is an unknown manufacturer'
FROM inserted i
WHERE NOT EXISTS
( SELECT 1
FROM Manufacturers m
WHERE m.Man_ID = i.Man_ID
);
INSERT dbo.Products
SELECT *
FROM inserted;
HOWEVER, I don't advocate this approach, I think it would be much better to just rely on the referencial integrity provided by the foreign key itself, and if the Man_ID does not exist let the insert fail, and if necessary ensure all Man_IDs exist before even attempting to insert.

Answer 2

I changed your trigger to following. In doing so, I made a few implicit assumptions. Assuming here that Products is a 3 column table (col1 & col2 being the other columns apart from Man_ID).
Hope this helps
alter
trigger checkman
on dbo.products
instead of insert
as
declare
#manid char(5),
#col1 varchar(50),
#transactionName varchar(20) = 'transaction1',
#col2 Varchar(500)
Begin
select #manid=Man_ID, #col1 = col1, #col2 = col2 from Inserted
IF (NOT EXISTS (SELECT Man_ID
FROM Manufacturers Where Man_ID = #manid
))
INSERT INTO ManuFacturers (Man_ID, Man_name, Man_Description) Values (#manid, 'Unknown Name', 'This is an unknown manufacturer')
INSERT INTO Products (Man_ID, col1, col2) values (#manid, #col1, #col2)
--begin try
-- /*begin tran #transactionName*/
-- Insert into Manufacturers (Man_ID, Man_Name, Man_Description) VALUES (#manid, 'Unknown Name', 'This is an unknown manufacturer')
-- insert into dbo.products select * from inserted;
--end try
--begin catch
-- /*rollback tran #transactionName;*/
-- insert into dbo.products select * from inserted;
--end catch
End