Rails - Apr 2006 - STI and through

I''m trying to understand how to do the following, but am finding myself
stuck.

I have three main tables; here are the first two:

CREATE TABLE reference_items (
  "id" integer primary key,
  "type" varchar(20),
  "title" varchar(255),
  "year" year(4)
);

CREATE TABLE "agents" (
  "id" integer primary key,
  "type" varchar(20),
  "sort_name" varchar(255), 
  "description" text
);

So I''ve got ReferenceItem and Agent classes, each of which has
subclasses (for
the first, stuff like Book, Article, etc.).

The third key table links the above:

CREATE TABLE contributors (
  "reference_item_id" integer,
  "agent_id" integer,
  "position" integer,
  "type" varchar(20)
);

The Contributor class currently has four subclasses: Author, Editor, Publisher,
and Translator. So the idea is I need to be able to separely track the type and
order of each class.

So how do I get this correctly setup in my models? Or can I?

Bruce