Rails - Nov 2005 - ActiveRecord & Inheritance

Hopefully a simple question...

I''ve got a table that, using the STI, contains some columns that are 
specific to one subclass or another.  How do I represent this fact in 
Ruby?  Specifically, in the following example, how do I keep someone 
from creating an instance of Classb and assigning a value to typeconly 
(an attribute that is only supposed to be applicable to objects of type 
Classc)?

DATABASE:

create table classas (
  id INTEGER AUTO_INCREMENT NOT NULL PRIMARY KEY,
  type VARCHAR(10) NOT NULL,
  genericattr VARCHAR(10) NOT NULL,
  typebonly INTEGER, -- for Classb instances only
  typeconly VARCHAR(10) -- for Classc instances only
);

RUBY:

class Classa < ActiveRecord::Base; end
class Classb < Classa
  # something that says attribute typebonly applies to this class only?
end
class Classc < Classa
  # something that says attribute typeconly applies to this class only?
end

Thanks.
-d

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