Hibernate: Parent/Child relationship in a single-table

Question

I hardly see any pointer on the following problem related to Hibernate. This pertains to implementing inheritance using a single database table with a parent-child relationship

Answer 1

I am not sure about you really want, but i think you want a Table per class hierarchy

In that case, each Entity is sorted by a DISCRIMINATOR_COLUMN as follows

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
    name="EMPLOYEE_TYPE", 
    discriminatorType = DiscriminatorType.STRING
)
@DiscriminatorValue("EMPLOYEE")
public class Employee {

    @Id @GeneratedValue
    @Column(name="EMPLOYEE_ID") 
    private Integer id = null;

}

And its Children is mapped according to

@Entity
@DiscriminatorValue("MANAGER")
public class Manager extends Employee {

    // Manager properties goes here        
     ...
}

In order to test, let's do the following

SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
Session session = sessionFactory.openSession();

/*
insert 
into
    Employee
    (EMPLOYEE_TYPE) 
values
    ('EMPLOYEE')
*/
session.save(new Employee());

/*
insert 
into
    Employee
    (EMPLOYEE_TYPE) 
values
    ('MANAGER')
*/
session.save(new Manager());

session.clear();
session.close();

But, instead of inheritance (which you can see a lot of NULL column due to more than one Entity share the same table - when using InheritanceType.SINGLE_TABLE strategy), your model would be better as follows

@Entity
public class Employee { 

    private Employee manager;
    private List reporteeList = new ArrayList();

    /**
    * optional=true
    * because of an Employee could not have a Manager
    * CEO, for instance, do not have a Manager
    */  
    @ManyToOne(optional=true)
    public Employee getManager() {
        return manager;
    }

    @OneToMany
    public List getReporteeList() {
        return reporteeList;
    }

}

Feel free to choice the best approach that fulfill your needs.

regards,