JPA - Composite key with @EmbeddedId involving @OneToOne relationship

We have already seen how to use @EmbeddedId on the basic types to map a composite key.

In this example of @EmbeddedId, we will see how to use an identity field of the target class (referenced class) of a @OneToOne relationship.

In this case (as compared to our basic example of @EmbeddedId; link given above), we will use @MapsId annotation on the referenced @OneToOne field instead of the @Id annotation, which will specify the name of the target entity's primary key field.

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

Example

@Entity
public class Task {
    @EmbeddedId
    private CompositeTaskId taskId;

    @MapsId("employeeKey")
    @OneToOne
    private Employee employee;

    private String taskName;
    private Date date;

    public Task() {
    }

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

@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;
    }
    .............
}

@Embeddable
public class CompositeTaskId implements Serializable{
    private long employeeKey;
    private long taskId;

    public CompositeTaskId() {
    }

    public CompositeTaskId(long employeeId, long taskId) {
        this.employeeKey = 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

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");
        CompositeTaskId cti = new CompositeTaskId(1L, 100L);
        Task task = new Task(cti, e);
        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 13:20:22.794, coding, 1]
-- Finding entity --
Task{taskId=CompositeTaskId{employeeKey=1, taskId=100}, employee=Employee{employeeId=1, name='Mike', dept='IT'}, taskName='coding', date=2025-10-29 13:20:22.794}

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