rails models - two tables have the same primary and foreign key fields

Question

I am using an existing database with a rails app.

I can't change the table or column names.

Lets say table one is "invoices" and table 2 is "orders"

they both have a primary key that is called the same thing, lets say "order_id"

invoices can find its order by looking at the primary key "order_id" in the orders table.

Vice versa for an order finding its invoice.

Invoices can have but not always have one order (you might be invoiced for something besides an order, like a "work_order" which would be found by looking for the "order_id" in the primary key position of the "work_orders" table. So invoices might have a work_order or an order.

orders always has an invoice work_orders always has an invoice

Im trying to figure out the classes in the models.

Do you set the primary and foreign keys to the same thing? What about belongs_to? The way this DB is set up, nothing really belongs to anything, they just reference each other by this same value "order_id". Would it be like this?

class Invoice < ActiveRecord::Base
    set_primary_key(:order_id)
    set_foreign_key(:order_id)
end

-- snip --

class Order < ActiveRecord::Base
    set_primary_key(:order_id)
    set_foreign_key(:order_id)
end

... And the same for a work order.

class WorkOrder < ActiveRecord::Base
    set_primary_key(:order_id)
    set_foreign_key(:order_id)
end

Is this correct? It seems to trashy of a way to do it but this DB is terrible.

What about all the belongs_to stuff?

Let me know if I have left anything out.

Thanks!