We illustrate generalization with the following example, which extends the information model of Part 4 by adding the object type
Employee and associating employees with publishers.
After adding the object type
Employee we notice that
Author share a number of attributes due to the fact that both employees and authors are people, and being an employee as well as being an author are roles played by people. So, we may generalize these two object types by adding a joint supertype
Person, as shown in the following diagram.
When generalizing two or more object types, we move those features that are shared by them to the newly added supertype where they are centralized. In the case of
Author, this set of shared features consists of the attributes
dateOfDeath. In general, shared features may include attributes, associations and constraints.
Notice that since in an information design model, each top-level class needs to have a standard identifier, in the new class
Person we have declared the standard identifier attribute
personId, which is inherited by all subclasses. Therefore, we have to reconsider the attributes that had been declared to be standard identifiers in the subclasses before the generalization. In the case of
Employee, we had declared the attribute
employeeNo as a standard identifier. Since the employee number is an important business information item, we have to keep this attribute, even if it is no longer the standard identifier. Because it is still an alternative identifier (a "key"), we define a uniqueness constraint for it with the constraint keyword
In the case of
Author, we had declared the attribute
authorId as a standard identifier. Assuming that this attribute represents a purely technical, rather than business, information item, we dropped it, since it's no longer needed as an identifier for authors. Consequently, we end up with a model which allows to identify employees either by their employee number or by their
personId value, and to identify authors by their
We consider the following extension of our original example model, shown in Figure 12.4, where we have added two class hierarchies: