Rule, update and aggregate functions

Hi,

I got stuck with the following problem and strangely couldn't find
anything similar in the list:

Assume two tables:
table: warehouse, columns: id, incoming, outgoing
table: articles, columns: id, articles_in_stock

All I want is to create a rule, that everytime when articles leave or
get in the warehouse, the sum of articles is updated in articles database.

Tried :

CREATE RULE upd_articles AS ON UPDATE TO warehouse
DO ALSO UPDATE articles SET articles_in_stock =SUM(NEW.incoming)-SUM
(NEW.outgoing)
 WHERE articles.id=OLD.id;

All I get is a "aggregate functions not allowed on update".

So I tried to wrap it in a SELECT:

CREATE RULE upd_articles AS ON UPDATE TO warehouse
DO ALSO UPDATE articles SET articles_in_stock =(SELECT SUM(NEW.incoming)-
SUM(NEW.outgoing)
FROM warehouse WHERE NEW.id=OLD.id)
 WHERE articles.id=OLD.id;

with the same result.

What is the right way to achieve my goal? Or are rules the wrong
instrument for it?

Any help is kindly appreciated,
Thanks, Sebastian