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R120 / 1c:1c-1

Association is formalized by zero or one Class

Class formalizes zero or one Class

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By 'formalize' we mean that an Association class holds the Attribute References necessary to map instances of the participating Classes in a given Association. Without an Association Class, an Association must be formalized by attribute references in one of its one or two participating Classes.

We model the rule that a Class may or may not participate as an Association Class on some Association, but if it does, that is the only Association where it may play that role. Put differently, a Class either is or is not an Association Class. If it is an Association Class, it serves this role on a single Association.

Note that a class that does play an Association Class role, may play roles in other Relationships as well. For example, a Class might simultaneously be a subclass of some Generalization while it serves as an Association Class on some Association. Or an Association Class may be a superclass or be a participating class in some othe Association. This is why we don't simply specialize Class into Association and 'Non-Association' Class.

A given Association may or may not be formalized by an Association Class. If it is, it is formalized by exactly one Association Class.

Formalization

Association Class.(Rnum, Domain) -> Association.(Rnum, Domain)
Association Class.(Class, Domain) -> Class.(Name, Domain)