9.3. Attributes
Like other ER notations, UML allows relationships to be modeled as attributes. For instance, in Figure 9.6(a) the Employee class has eight attributes. The corresponding ORM diagram is shown in Figure 9.6(b).
Figure 9.6. UML attributes (a) depicted as ORM relationship types (b).
In UML, attributes are mandatory and single valued by default. So the employee number, name, title, gender, and smoking status attributes are all mandatory. In the ORM model, the unary predicate “smokes” is optional (not everybody has to smoke). UML does not support unary relationships, so it models this instead as the Boolean attribute “isSmoker”, with ...
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