JPA - OneToMany/ManyToOne bidirectional example for persisting Map with basic type keys and entity values

Just like OneToMany/ManyToOne bidirectional mapping of other collections, java.util.Map entity values have a reverse reference of this entity with @ManyToOne annotation. Also we need to use 'mappedBy' element of @OneToMany annotation.

Example

@Entity
public class Employee {
  @Id
  @GeneratedValue
  private long id;
  private String name;
  @OneToMany(mappedBy = "taskEmployee", cascade = CascadeType.ALL)
  @MapKeyColumn(name = "TASK_DATE", nullable = true)
  private Map<Date, Task> tasks;
    .............
}

@Entity
public class Task {
  @Id
  @GeneratedValue
  private long id;
  private String name;
  private String description;
  @ManyToOne
  @JoinColumn(name = "EMP_FK")
  private Employee taskEmployee;
    .............
}

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 EMPLOYEE");
          nativeQuery(em, "SHOW COLUMNS from TASK");
      } 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 EMPLOYEE'
[ID, BIGINT(19), NO, PRI, NULL]
[NAME, VARCHAR(255), YES, , NULL]
'SHOW COLUMNS from TASK'
[ID, BIGINT(19), NO, PRI, NULL]
[DESCRIPTION, VARCHAR(255), YES, , NULL]
[NAME, VARCHAR(255), YES, , NULL]
[EMP_FK, BIGINT(19), YES, , NULL]
[TASK_DATE, TIMESTAMP(23), YES, , NULL]

Persisting and loading data

public class ExampleMain2 {

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

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

      Employee e1 = new Employee();
      e1.setName("employee name 1");

      Task task1 = new Task();
      task1.setName("task 1");
      task1.setDescription("task 1 desc");
      task1.setTaskEmployee(e1);

      Task task2 = new Task();
      task2.setName("task 2");
      task2.setDescription("task 2 desc");
      task2.setTaskEmployee(e1);

      SimpleDateFormat sdf = new SimpleDateFormat("MM-dd-yy");
      e1.addTask(sdf.parse("15-01-17"), task1);
      e1.addTask(sdf.parse("20-01-17"), task2);
      System.out.println(e1);

      Employee e2 = new Employee();
      e2.setName("employee name 2");

      Task task3 = new Task();
      task3.setName("task 3");
      task3.setDescription("task 3 desc");
      task3.setTaskEmployee(e2);

      e2.addTask(sdf.parse("02-05-2017"), task3);
      System.out.println(e2);

      em.getTransaction().begin();
      em.persist(e1);
      em.persist(e2);

      em.getTransaction().commit();
      em.close();
  }

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

  private static void loadEntity(EntityManagerFactory emf) {
      System.out.println("-- Loading entities --");
      EntityManager em = emf.createEntityManager();
      List<Employee> entityAList = em.createQuery("Select t from Employee t")
                                     .getResultList();
      entityAList.forEach(System.out::println);
      em.close();
  }
}

Output

-- Persisting entities --
Employee{id=0, name='employee name 1', tasks={Thu Mar 01 00:00:00 CST 2018=Task{id=0, name='task 1', description='task 1 desc', taskEmployee=0}, Wed Aug 01 00:00:00 CDT 2018=Task{id=0, name='task 2', description='task 2 desc', taskEmployee=0}}}
Employee{id=0, name='employee name 2', tasks={Sun Feb 05 00:00:00 CST 2017=Task{id=0, name='task 3', description='task 3 desc', taskEmployee=0}}}
-- Native queries --
'Select * from Employee'
[1, employee name 1]
[4, employee name 2]
'Select * from Task'
[2, task 1 desc, task 1, 1, 2018-03-01 00:00:00.0]
[3, task 2 desc, task 2, 1, 2018-08-01 00:00:00.0]
[5, task 3 desc, task 3, 4, 2017-02-05 00:00:00.0]
-- Loading entities --
Employee{id=1, name='employee name 1', tasks={2018-03-01 00:00:00.0=Task{id=2, name='task 1', description='task 1 desc', taskEmployee=1}, 2018-08-01 00:00:00.0=Task{id=3, name='task 2', description='task 2 desc', taskEmployee=1}}}
Employee{id=4, name='employee name 2', tasks={2017-02-05 00:00:00.0=Task{id=5, name='task 3', description='task 3 desc', taskEmployee=4}}}

In above example the foreign-key and map key columns are mapped to the corresponding table of the value entity. If we want to map an intermediate join table instead, then we have to use @JoinTable on both sides. Please see a similar example here.

Example Project

Dependencies and Technologies Used:

• h2 1.4.195: 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