Thanks for the reply.
Sorry if I'm not (yet) understand it. I understand the example from the ORM manual: Age is a calculated field, so you can extract it from e.g. a birthday field and there is no need to save it.
In the ProductsModule the field 'WhatsInTheBox' is a textfield you can fill in. Does it mean that this value isn't saved in the database? How does it get its value then?