JPA - Derived Composite key (The parent entity uses @IdClass)

The derived identities may be captured by means of of composite primary. In this example, we will demonstrate how the @ManyToOne target entity will use @IdClass to define a composite primary key.

Example

@Entity
@IdClass(CompositeTaskId.class)
public class Task {
    @Id
    private long taskId;

    @Id
    @ManyToOne
    @JoinColumns({
            @JoinColumn(name = "FK_firstName", referencedColumnName = "firstName"),
            @JoinColumn(name = "FK_lastName", referencedColumnName = "lastName")
    })
    private Employee employee;

    private String taskName;
    private Date date;

    public Task() {
    }

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

Note that using @JoinColumns in above class, is optional. If we don't use it then JPA uses the default naming strategy for foreign key columns.

Following class which is the target of @ManyToOne relationship (also called 'parent' from database perspective as it will correspond to a foreign key referenced table), will define a composite primary key using @IdClass:

@Entity
@IdClass(EmployeeId.class)
public class Employee {
    @Id
    private String firstName;
    @Id
    private String lastName;
    private String dept;

    public Employee() {
    }

    public Employee(String firstName, String lastName, String dept) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.dept = dept;
    }
    .............
}

public class EmployeeId implements Serializable{
    private String firstName;
    private String lastName;

    public EmployeeId() {
    }

    public EmployeeId(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }
    .............
}

public class CompositeTaskId implements Serializable {
    private EmployeeId employee;
    private long taskId;

    public CompositeTaskId() {
    }

    public CompositeTaskId(EmployeeId employee, long taskId) {
        this.employee = employee;
        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]
[FK_FIRSTNAME, VARCHAR(255), NO, PRI, NULL]
[FK_LASTNAME, VARCHAR(255), NO, PRI, NULL]
'SHOW COLUMNS from EMPLOYEE'
[FIRSTNAME, VARCHAR(255), NO, PRI, NULL]
[LASTNAME, VARCHAR(255), NO, PRI, NULL]
[DEPT, 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( "Darion"," Schaden", "IT");

        Task task1 = new Task(e, 100L);
        task1.setTaskName("Coding");
        task1.setDate(new Date());

        Task task2 = new Task(e, 200L);
        task2.setTaskName("Refactoring");
        task2.setDate(new Date());

        em.getTransaction().begin();
        em.persist(e);
        em.persist(task1);
        em.persist(task2);
        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();
        EmployeeId employeeId = new EmployeeId("Darion", " Schaden");
        CompositeTaskId taskId = new CompositeTaskId(employeeId, 100);
        Task task = em.find(Task.class, taskId);
        System.out.println(task);
        em.close();
    }
}

Output

-- Persisting entity --
-- Native query --
'Select * from EMPLOYEE'
[Darion,  Schaden, IT]
'Select * from Task'
[100, 2025-10-29 13:21:22.08, Coding, Darion,  Schaden]
[200, 2025-10-29 13:21:22.08, Refactoring, Darion,  Schaden]
-- Finding entity --
Task{taskId=100, employee=Employee{firstName='Darion', lastName=' Schaden', dept='IT'}, taskName='Coding', date=2025-10-29 13:21:22.08}

Example Project

Dependencies and Technologies Used:

• h2 1.4.193: H2 Database Engine.
• hibernate-core 5.2.8.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