An Association rule is used to define a relationship between any two classes, based on values of properties in the classes. An example is Association rule pxAssignedOperatorInfo defined in Work-, which relates each instance to an assignment instance in Assign- by comparing pxAssignedOperatorID in Work- to pyUserIdentifier in Assign-. These relationships are inherited by descendent classes of the classes on which the Association is defined; in this example, all descendent classes of Work- and Assign-. The primary use of Association rules is in Report Definition rules, where they are used to add an appropriate JOIN to the automatically-generated SQL for a query defined on the Association’s class when that query also references properties in the associated class. Association rules are one-directional, and only apply when the primary class of a Report Definition rule is the same as, or is a descendent of, the primary class of the Association rule. More than one Association rule may be defined between two classes, if there are multiple ways of relating or joining instances of the two classes.