Enterprise JavaBeans 3.1, 6th Edition

Chapter 11. Entity Relationships

Chapter 10 covered basic persistence mappings, including various ways to define primary keys as well as simple and complex property-type mappings. This chapter retools our employee registry a bit further by discussing the relationships between entities.

In order to model real-world business concepts, entity beans must be capable of forming relationships. For instance, an employee may have an address; we’d like to form an association between the two in our database model. The address could be queried and cached like any other entity, yet a close relationship would be forged with the Employee entity. Entity beans can also have one-to-many, many-to-one, and many-to-many relationships. For example, the Employee entity may have many phone numbers, but each phone number belongs to only one employee (a one-to-many relationship). Similarly, an employee may belong to many teams within his or her organization, and teams may have any number of employees (a many-to-many relationship).

The Seven Relationship Types

Seven types of relationships can exist between entity beans. There are four types of cardinality: one-to-one, one-to-many, many-to-one, and many-to-many. In addition, each relationship can be either unidirectional or bidirectional. These options seem to yield eight possibilities, but if you think about it, you’ll realize that one-to-many and many-to-one bidirectional relationships are actually the same thing. Thus, there are only seven distinct relationship ...