In this paper, we consider the framework of "complex" information systems,
related for example to biology, geographical information, or multimedia.
Apart from being inherently complex, objects of these domains are evolutive,
and related applications require more and more often to take into account
this functionality.
In traditional class-based models, an object is linked to a single class,
according to the mono-instanciation principle, and this class determines the
object's structure and behaviour during its lifespan.
In this paper, we present an approach for extending the UML formalism, in
order to take into account evolution of objects. The extension we propose is
based on the main concepts of the database model so-called Samovar, which is
a fully ODMG-compliant object-role database model. We point out that the
 role notion presented here is different of  the one appearing in release
 2.0 of UML.