Multilevel Class inheritance

Hello there,

I am having a problem inheriting an already inherited class.

errr.. sounds more complicated than it is.. look:

class Item < ActiveRecord::Base
# has a name and description field
end

class Reference < Item
# has a something field
end

class UrlReference < Reference
#has an url field
end

... now If I do make a u = UrlReference.new in the console for
example, the u instance does NOT have the url field.. only the ones
from Reference and Item...

How come? I guess that's basic ruby.. but how do I properly inherit
from other classes which again inherit from other ones? Did I miss
something here?

Cheers & thanks,
-Joerg