Sometimes a group of people act as a single unit in a use case, or, more generally, a phenomenon is composed of other
independent phenomena. For example, School Class consists of Students. Such a phenomenon is called an aggregate.
Aggregates are modeled with a separate class for the composite phenomenon. Such classes have aggregations to the
classes that represent its constituents. This construction makes it possible to both refer to the components
individually and handle them as a single unit. The uniting class does not necessarily have many properties of its own.
Its essential characteristic may very well be the aggregations of the different components.
A company's board of directors consists of the chairman, the chief executive officer, and several owner
An aggregate class holds other classes together.
See also Guideline: Aggregation for more general information.
You should use aggregates only if they are necessary; that is, if both the aggregate and any of its constituents are
supposed to act or be useful on their own. A good aggregate is a natural, coherent part of a business analysis
model-its meaning should be easy to understand from the context.
Aggregations should only be used with classes representing the same kind of phenomenon. For example, it does not make
sense for a business entity to be an aggregate of business workers.