All my business objects have an IsValid property to check on mandatory fields etc. Before saving these objects to the database I want all objects to be valid. But when calling the Flush method of the session, obviously this IsValid property is ignored. How can I assure that NHibernate is only persisting valid objects?

Current NH version is 1.0.2

Either create an interceptor (a special class implementing IInterceptor) and do the check in its methods, or make your objects implement IValidatable, then their Validate method will be called when they are being persistent.

The second solution is a bit more intrusive than the first and is not recommended.