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Terry Halpin
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Monday, 13 June 2005 |
Some information modeling approaches allow instances of relationships
to be treated as entities in their own right. In the Unified Modeling Language
(UML), this is called “reification”, and is mediated by association
classes. In Object-Role Modeling (ORM), this is called “objectification” or
“nesting”. While this modeling option is rarely supported by industrial versions
of Entity-Relationship Modeling (ER), some academic ER versions do support
it. Objectification is related to the linguistic activity of nominalization, of which
two flavors may be distinguished: circumstantial; and propositional. In practice,
objectification is prone to misuse, and some modeling approaches provide incomplete
or flawed support for it. This paper analyzes objectification in-depth,
shedding new light on its fundamental nature, and providing practical guidelines
on using objectification to model information. Because of its richer semantics,
the main graphic notation used is that of ORM. However, the main
ideas are relevant to UML and ER as well.
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