First-class relationships in an object-oriented language

By Gavin Bierman and Alisdair Wren

"In this paper we investigate the addition of first-class relationships to a prototypical object-oriented programming language (a “middleweight” fragment of Java). We provide language-level constructs to declare relationships between classes and to manipulate relationship instances. We allow relationships to have attributes and provide a novel notion of relationship inheritance. ..."
http://homepages.inf.ed.ac.uk/wadler/fool/program/final/4/4_Paper.pdf