1.3. Part 2 - Mapping associations
We mapped a persistent entity class to a table. Let's build on this and add some class associations. First we'll add people to our application, and store a list of events they participate in.
1.3.1. Mapping the Person class
The first cut of the Person class is simple:
package events;
public class Person {
private Long id;
private int age;
private String firstname;
private String lastname;
public Person() {}
// Accessor methods for all properties, private setter for 'id'
}
Create a new mapping file called Person.hbm.xml (don't forget the DTD reference at the top):
<hibernate-mapping>
<class name="events.Person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="native"/>
</id>
<property name="age"/>
<property name="firstname"/>
<property name="lastname"/>
</class>
</hibernate-mapping>
Finally, add the new mapping to Hibernate's configuration:
<mapping resource="events/Event.hbm.xml"/>
<mapping resource="events/Person.hbm.xml"/>
We'll now create an association between these two entities. Obviously, persons can participate in events, and events have participants. The design questions we have to deal with are: directionality, multiplicity, and collection behavior.
