Guidelines: Aggregation in
the Business Analysis Model
Topics
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.
Example:
A company's board of directors consists of the chairman, the chief
executive officer, and several owner representatives.
An aggregate class holds other classes
together.
See also Guidelines: 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.
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