Creation and validation of object instances with has_many and has_one

Hello everybody,

I am slowly beginning to understand some principles behind Ruby and
Rails and am working on my first project. It's basically this:

Users register for an event, with personal information, preferences
and so on. Then the backend office modifies their data based on
whether they have paid (for example).

For example every user can register as a normal visitor, volunteer,
technician or several other options. So I have

class User << AR::Base