Modeling Collections in UML and ORM

Collection types such as sets, bags and arrays have been used as data structures in both traditional and object oriented programming. Although sets were used as record components in early database work, this practice was largely discontinued with the widespread adoption of relational databases. Object-relational and object databases once again allow database designers to embed collections as database fields. Should collections be specified directly on the conceptual schema, as mapping annotations to the conceptual schema, or only on the logical database schema? This paper discusses the pros and cons of different approaches to modeling collections. Overall it favors the annotation approach, whereby collection types are specified as adornments to the pure conceptual schema to guide the mapping process from conceptual to lower levels. The ideas are illustrated using notations from both object-oriented (Unified Modeling Language) and fact-oriented (Object-Role Modeling) approaches.