These three classes are domain-specific; references to their instances flow down wires. The left-hand two, Category and Item, are copies of the external relational database. The OrderItem class, which is derived from the Item class by adding three fields, is treated the same but its instances are created internally. The three new fields in the OrderItem record are shown in italics. The Quantity field is live; that is, when you store into it the ExtendedPrice value is automatically computed.