Referencing exactly two models of the same kind

Hi,

how do I solve this in Rails: Lets say I have a User and a Message.
Now, a Message is sent from one User to another User, which means that
in the Message I have to reference a User twice!

This seems to be a problem, if I do not user one-to-many or many-to-
many relationships, because it seems to me as if I could reference a
Model of the same type only once.

However, I would like the Message model to contain 2 User fields. So
this would look something like this:

class User ActiveRecord::Base
# Given a User, I do not require to be able to retrieve all his
messages.
end

class Message < ActiveRecord::Base
belongs_to :user; # Sender
belongs_to :user2; # Receiver
end

The DB Tables should look something like this:

users
id

messages
id, user_id, user2_id;

How do the Rails models and the database tables look like? I'm
generally wondering how to reference more than one model of the same
kind from within another model, without using many_to_many or
one_to_many relationships.

Fritz

I once wrote a blog post that explains this: http://www.spacevatican.org/2008/5/6/creating-multiple-associations-with-the-same-table

Fred