function/trigger problems

I am trying to update a timestamp value in a related table using
functions and triggers, but failing miserably! I'd be grateful for some
help.

I'm trying to update the tstamp value in a whenever an insert is done
on b where b.a_id = a.id. So
INSERT INTO b (a_id) values (1)
should update the tstamp value in a using the trigger.
My function is failing at the 'WHERE' clause.

-----------------------------------------------------------------------

CREATE TABLE "a" (
"id" SERIAL,
"tstamp" timestamp
);

CREATE TABLE "b" (
"id" SERIAL,
"a_id" integer,
"tstamp" timestamp default current_timestamp.
);

CREATE FUNCTION "fnTU" () RETURNS opaque AS 'BEGIN
UPDATE a SET tstamp = new.tstamp;
WHERE new.a_id = id;
RETURN new;
END' LANGUAGE 'plpgsql';

CREATE TRIGGER "trgTU" AFTER INSERT ON "b" FOR EACH ROW EXECUTE PROCEDURE "fnTU" ();

COPY "a" FROM stdin;
1 2002-06-18 09:47:44.734527+01
2 2002-06-18 09:47:44.734527+01

COPY "b" FROM stdin;
1 1 2002-06-18 09:47:44.734527+01

--
Rory Campbell-Lange
<rory(at)campbell-lange(dot)net>
<www.campbell-lange.net>