Using a rule as a trigger.

Hi all,

I have looked at some previous posting and thought that I had found 
exactly what I need. What I need is to insert an id (from a sequence) 
when a new record is inserted. The following example almost does what I 
need :

CREATE TABLE  topics (id int, topic varchar(50), descriotion text);
CREATE SEQUENCE nextid start 1;
CREATE RULE ins_topic AS ON INSERT TO topics WHERE id ISNULL DO UPDATE 
topics SET id=nextval('nextid') WHERE  id ISNULL;


This example updates the last insert. I need it to update the currnet 
insert. How do I do this?

Kind regards
Andrew Higgs