Is it possible for the developer not to generate any code but just define the mapping between the entity classes (that he has written manually) and DB tables? I had a little experience (just tried a simple mapping) with NHibernate and didn't generate any code. Just filled a meta-data file about the mapping. Is it possible to generate nothing but just define the mapping logic?

That's called the 'POCO' approach and we currently don't support that. POCO requires class generation at runtime or post-compile IL manipulation. NHibernate generates at runtime proxy classes, we do that at generation time, so instead of getting them at runtime you get them earlier.