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>One other thing I've been confused on: in a current project I'd like to model, we have an order table, an order line table, a payment table, and a payment line table. The payment line table refers to the invoice/order. That's four classes as well, right? with Orders and Payments having collection Properties containing line items?

That's how I would do it. Have data objects for each table. Then have a business object for say, orders that has all the smarts needed. The UI designer would not need to call the data objects, just the orders object.