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

You are expressing two concepts here:

1. inheritance and you want to map your inheritance hierarchy in a single table.
2. a parent/child relationship.

To implement 1., you'll need to use Hibernate's single table per class hierarchy strategy:

@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(
    name="emptype",
    discriminatorType=DiscriminatorType.STRING
)
public abstract class Employee { ... }

@Entity
@DiscriminatorValue("MGR")
public class Manager extends Employee { ... }

To implement 2., you'll need to add two self-referencing associations on Employee:

• many employee have zero or one manager (which is also an Employee)
• one employee has zero or many reportee(s)

The resulting Employee may looks like this:

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

    ... 

    private Employee manager;
    private Set reportees = new HashSet();

    @ManyToOne(optional = true)
    public Employee getManager() {
        return manager;
    }

    @OneToMany
    public Set getReportees() {
        return reportees;
    }

    ...
}

And this would result in the following tables:

CREATE TABLE EMPLOYEE_EMPLOYEE (
        EMPLOYEE_ID BIGINT NOT NULL,
        REPORTEES_ID BIGINT NOT NULL
    );

CREATE TABLE EMPLOYEE (
        EMPTYPE VARCHAR(31) NOT NULL,
        ID BIGINT NOT NULL,
        NAME VARCHAR(255),
        MANAGER_ID BIGINT
    );

ALTER TABLE EMPLOYEE ADD CONSTRAINT SQL100311183749050 PRIMARY KEY (ID);

ALTER TABLE EMPLOYEE_EMPLOYEE ADD CONSTRAINT SQL100311183356150 PRIMARY KEY (EMPLOYEE_ID, REPORTEES_ID);

ALTER TABLE EMPLOYEE ADD CONSTRAINT FK4AFD4ACE7887BF92 FOREIGN KEY (MANAGER_ID)
    REFERENCES EMPLOYEE (ID);

ALTER TABLE EMPLOYEE_EMPLOYEE ADD CONSTRAINT FKDFD1791F25AA2BE0 FOREIGN KEY (REPORTEES_ID)
    REFERENCES EMPLOYEE (ID);

ALTER TABLE EMPLOYEE_EMPLOYEE ADD CONSTRAINT FKDFD1791F1A4AFCF1 FOREIGN KEY (EMPLOYEE_ID)
    REFERENCES EMPLOYEE (ID);