JPA - Composite key with @IdClass involving @OneToOne relationship (Derived Identity)

We have already seen how to use @IdClass to map a composite key, but that use was limited to the identity of basic supported types.

In this example of @IdClass, we will see how to use an identity field of the target class (referenced class) of a @OneToOne relationship. The identity field of the target class is referred as 'derived identity field'.

The only difference, in this case (as compared to our last example of @IdClass; link given above) is, we will place one of our @Id annotations on the referenced @OneToOne field instead of some basic type field.

Let's see the example to understand how that works.

Example

@Entity
@IdClass(CompositeTaskId.class)
public class Task {
    @Id
    private long taskId;
    @Id
    @OneToOne
    private Employee employee;
    private String taskName;
    private Date date;

    public Task() {
    }

    public Task(Employee employee, long taskId) {
        this.employee = employee;
        this.taskId = taskId;
    }
    .............
}

@Entity
public class Employee {
    @Id
    private long employeeId;
    private String name;
    private String dept;

    public Employee() {
    }

    public Employee(long employeeId, String name, String dept) {
        this.employeeId = employeeId;
        this.name = name;
        this.dept = dept;
    }
    .............
}

public class CompositeTaskId implements Serializable {
    //the names of the both fields should be same as the @Id fields in source class
    //the type of 'taskId' field is same as of the basic @Id field defined in the Task entity
    private long taskId;
    //the type of 'employee' field is same as of the @Id field defined in Employee entity.
    private long employee;

    public CompositeTaskId() {
    }

    public CompositeTaskId(long employeeId, long taskId) {
        this.employee = employeeId;
        this.taskId = taskId;
    }
    .............
}

Main class showing table mappings

public class ExampleMain {

    public static void main(String[] args) {
        EntityManagerFactory emf =
                Persistence.createEntityManagerFactory("example-unit");
        try {
            EntityManager em = emf.createEntityManager();
            nativeQuery(em, "SHOW TABLES");
            nativeQuery(em, "SHOW COLUMNS from TASK");
            nativeQuery(em, "SHOW COLUMNS from EMPLOYEE");
        } finally {
            emf.close();
        }
    }

    public static void nativeQuery(EntityManager em, String s) {
        System.out.printf("'%s'%n", s);
        Query query = em.createNativeQuery(s);
        List list = query.getResultList();
        for (Object o : list) {
            if (o instanceof Object[]) {
                System.out.println(Arrays.toString((Object[]) o));
            } else {
                System.out.println(o);
            }
        }
    }
}

Output

'SHOW TABLES'
[EMPLOYEE, PUBLIC]
[TASK, PUBLIC]
'SHOW COLUMNS from TASK'
[TASKID, BIGINT(19), NO, PRI, NULL]
[DATE, TIMESTAMP(23), YES, , NULL]
[TASKNAME, VARCHAR(255), YES, , NULL]
[EMPLOYEE_EMPLOYEEID, BIGINT(19), NO, UNI, NULL]
'SHOW COLUMNS from EMPLOYEE'
[EMPLOYEEID, BIGINT(19), NO, PRI, NULL]
[DEPT, VARCHAR(255), YES, , NULL]
[NAME, VARCHAR(255), YES, , NULL]

H2 database SHOW statements

In above output, note that EMPLOYEE_EMPLOYEEID column is UNI (unique), which is important for a @OneToOne relationship. In case of @ManyToOne, this column is not unique (that's the only difference between @OneToMany and @ManyToOne) which allows to reference the multiple target entity instances for a same source entity. Check out our @OneToOne and @ManyToOne tutorials if not already familiar with the concepts.

Persisting and loading data

public class ExampleMain2 {

    public static void main(String[] args) throws Exception {
        EntityManagerFactory emf =
                Persistence.createEntityManagerFactory("example-unit");
        try {
            persistEntity(emf);
            runNativeQuery(emf);
            findEntityById(emf);
        } finally {
            emf.close();
        }
    }

    private static void persistEntity(EntityManagerFactory emf) throws Exception {
        System.out.println("-- Persisting entity --");
        EntityManager em = emf.createEntityManager();

        Employee e = new Employee(1L, "Mike", "IT");
        Task task = new Task(e, 100L);
        task.setTaskName("coding");
        task.setDate(new Date());

        em.getTransaction().begin();
        em.persist(e);
        em.persist(task);
        em.getTransaction().commit();
        em.close();
    }

    private static void runNativeQuery(EntityManagerFactory emf) {
        System.out.println("-- Native query --");
        EntityManager em = emf.createEntityManager();
        ExampleMain.nativeQuery(em, "Select * from EMPLOYEE");
        ExampleMain.nativeQuery(em, "Select * from Task");
    }

    private static void findEntityById(EntityManagerFactory emf) {
        System.out.println("-- Finding entity --");
        EntityManager em = emf.createEntityManager();
        CompositeTaskId taskId = new CompositeTaskId(1, 100);
        Task task = em.find(Task.class, taskId);
        System.out.println(task);
        em.close();
    }
}

Output

-- Persisting entity --
-- Native query --
'Select * from EMPLOYEE'
[1, IT, Mike]
'Select * from Task'
[100, 2025-10-29 13:20:02.777, coding, 1]
-- Finding entity --
Task{taskId=100, employee=Employee{employeeId=1, name='Mike', dept='IT'}, taskName='coding', date=2025-10-29 13:20:02.777}

Example Project

Dependencies and Technologies Used:

• h2 1.4.196: H2 Database Engine.
• hibernate-core 5.2.10.Final: The core O/RM functionality as provided by Hibernate.
  Implements javax.persistence:javax.persistence-api version 2.1
• JDK 1.8
• Maven 3.3.9