Hello Mikko,
As Ryan mentioned, you have to use (:or db-null integer) to specify a regular integer field, since it will default to "NOT NULL". Also, if you are just starting to use the library, it is probably a better idea to get the current code from the darcs repository -- I'm about to release a new version with a slightly changed interface, and one of the changes is that the defclass macro is replaced by a dao-class metaclass, which handles unbound slots in a much nicer way. The repository contains docs which are updated for that version, and is not expected to change much before the 1.10 release. (I don't usually break backwards compatibility, but there were a few issues that required cleaning up, so 1.10 will.)
On the subject of 1.10 -- has anyone apart from Attila tried the current code yet?
Cheers, Marijn