Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and the Unified Process, Second Edition

12.5. Design Creep: No Attributes as Foreign Keys

Attributes should not be used to relate conceptual classes in the domain model. The most common violation of this principle is to add a kind of foreign key attribute, as is typically done in relational database designs, in order to associate two types. For example, in Figure 12.5 the currentRegisterNumber attribute in the Cashier class is undesirable because its purpose is to relate the Cashier to a Register object. The better way to express that a Cashier uses a Register is with an association, not with a foreign key attribute. Once again, relate types with an association, not with an attribute.

Figure 12.5. Do not use attributes as foreign keys.

There are many ways to relate objects—foreign ...