Objectification of Relationships

Abstract

Some popular information-modeling approaches allow instances of relationships or associations to be treated as entities in their own right. Object-role modeling (ORM) calls this process “objectification” or “nesting.” In the unified modeling language (UML), this modeling technique is called “reification,” and is mediated by means of association classes. While this modeling option is rarely supported by industrial versions of entity-relationship modeling (ER), it is allowed in several academic versions of ER. Objectification is related to the linguistic activity of nominalization, of which two flavors may be distinguished: situational and propositional. In practice, objectification needs to be used judiciously, as its misuse can lead to implementation anomalies, and those modeling approaches that permit objectification often provide incomplete or flawed support for it. This chapter provides an in-depth analysis of objectification, shedding new light on its fundamental nature, and providing practical guidelines on using objectification to model information systems. Because of its richer semantics, the main graphic notation used is that of ORM 2 (the latest generation of ORM); however, the main ideas are relevant to UML and ER as well.