It is common in starting models to create the entity class Employee. This is a classic example of the flaw in data modeling that confuses the nature of a thing with the role that it plays. Employees are fundamentally people. The entity class of interest, then, must be Person, which we’ve already described in detail in Chapter 4. Clearly a person is not born an employee. Indeed, over the course of h’[(] life, ‘e may well play the role of being an employee in more than one company.

We need a different approach.

In fact, the Role of being an employee, as played by a Person for a Company is itself a thing of significance—albeit an abstract one. In Figure 13-1, this is shown by the entity class Employment, which is defined as “the fact ...

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