Rails 3.1 with PostgreSQL: GROUP BY must be used in an aggregate function

I am trying to load the latest 10 Arts grouped by the user_id and ordered by created_at. This works fine with SqlLite and MySQL, but gives an error on my new PostgreSQL database.

Art.all(:order => "created_at desc", :limit => 10, :group => "user_id")

ActiveRecord error:

Art Load (18.4ms)  SELECT "arts".* FROM "arts" GROUP BY user_id ORDER BY created_at desc LIMIT 10
ActiveRecord::StatementInvalid: PGError: ERROR:  column "arts.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT  "arts".* FROM "arts"  GROUP BY user_id ORDER BY crea...

Any ideas?