A data model that allows for relationships between entities, also referred to as parties, to be modeled as attributes of an entity and for customization of the data model in a manner that facilitates upgrading of the data model. The data model defines a party class that includes a party identifier and a list of relationships of that party with other parties. The relationships may include represented-by relationships, customer-of relationships, contact-of relationships, or employee-of relationships. The party class can be sub-classed (i.e., be a base class for a derived class) depending on the type of party that is being model.