CODEWITHSUNDEEP
uml
Har
Har

Reputation: 3918

UML - Multiple decomposition relationships between two entities

I want to model an airport and a city and would like to choose the appropriate decomposition relationship between the two but I cant seem to make up my mind which one to choose as they both have different relationships.

A City can exist on its own and does not require an airport hence I would say that City-Airport link is an aggregation however...

An Airport requires a City and will not exists without one making it a Composition relationship.

Is it possible to have two relationships? One which is City -> Airport and the other being Airport -> City it does feel strange however since a Composition to me feels like a relationship which should be honoured by two people and not just one.

Upvotes: 2

Views: 322

Answers (1)

Gerd Wagner
Gerd Wagner

Reputation: 5673

It's a widespread misunderstanding that the relationships between classes are primarily either "aggregations" or "compositions". I think this unfortunate tradition has been extablished in the C++ community.

Rather, the most important kind of relationships between classes are "associations", as they are called in UML class models/diagrams. So, if you want to model the relationship between Cityand Airport, you probably want to choose a one-to-many association (since a city may have more than one airport, but any airport is assigned to a city).

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In UML, both aggregations and compositions are special cases of associations, used for expressing a part-whole relationship between the instances of both classes. Since an airport is not really a part of a city, but just related to it, there is neither a Composition nor an Aggregation between Cityand Airport, but just a plain Association.

In many cases, where we may wonder if an association is a composition, it is safer to model it as a plain association.

The only good reason for modeling an association as a composition is when the instances of the component type are "weak entities" not having their own identity (object ID). But airports do have their own ID, so there is no need, and no gain, to model them as components of a city.

Upvotes: 3

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