Hibernate extend entity for same table

Question

I have a table with two fields I would like to have two objects.

First one only has field1

@Entity(name = \"simpleTableObject\")
@Table(name = \"someTabl         


        

Answer 1

I was able to do something about your question. I defined another class hierarchy: UserWithRole that extends User which is similar to yours.

Define class User as an entity with inheritance strategy SINGLE_TABLE:

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@Table(name = "USERS")
public class User {
 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
 protected Long id;
 @Column(nullable = false)
 protected String name;
 // toString(), default constructor, setters/getters, more constructors.
 ...
}

This inheritance strategy has one considerable disadvantage:

• Subclasses can't contain non-nullable columns.

There is another strategy JOINED that allows creating non-nullable columns in subclasses. It creates an additional table for each subclass these tables have FK to the superclass table.

Define UserWithRole class:

@Entity
public class UserWithRole extends User {
  private String role;
  // toString(), default constructor, setters/getters, more constructors.
 ...
}

Add Helper class to create users in the database and use your query:

@Component
public class Helper {
 @Autowired
 EntityManager entityManager;
 @Transactional
 public void createUsers() {
  for (long i = 0; i < 10; i++) {
   User user;
   if (i % 2 == 0) {
    user = new UserWithRole("User-" + i, "Role-" + i);
   } else {
    user = new User("User-" + i);
   }
   entityManager.persist(user);
  }
  entityManager.flush();
 }

 @Transactional(readOnly = true)
 @SuppressWarnings("unchecked")
 public < T > List < T > get(Class < T > aClass) {
  SessionFactory sessionFactory = entityManager.getEntityManagerFactory().unwrap(SessionFactory.class);
  ClassMetadata hibernateMetadata = sessionFactory.getClassMetadata(aClass);
  if (hibernateMetadata == null) {
   return null;
  }
  if (hibernateMetadata instanceof AbstractEntityPersister) {
   AbstractEntityPersister persister = (AbstractEntityPersister) hibernateMetadata;
   String entityName = persister.getEntityName();

   if (entityName != null) {
    return sessionFactory.getCurrentSession().
    createQuery("from " + entityName).list();
   }
  }
  return null;
 }

}

As you may see, I changed your method a bit:

• Added generic type to avoid unsafe type casts;
• Used entity name instead of table name because HQL expects entity name.

Let's start testing

Get all User instances:

@Test
public void testQueryUsers() {
 helper.createUsers();
 for (User user: helper.get(User.class)) {
  System.out.println(user);
 }
}

Output (users with role are still UserWithProfile instances at runtime):

UserWithRole{id=1, name='User-0', role='Role-0'}
User{id=2, name='User-1'}
UserWithRole{id=3, name='User-2', role='Role-2'}
User{id=4, name='User-3'}
UserWithRole{id=5, name='User-4', role='Role-4'}
User{id=6, name='User-5'}
UserWithRole{id=7, name='User-6', role='Role-6'}
User{id=8, name='User-7'}
UserWithRole{id=9, name='User-8', role='Role-8'}
User{id=10, name='User-9'}

SQL query issued by Hibernate:

select
user0_.id as id2_0_,
 user0_.name as name3_0_,
 user0_.role as role4_0_,
 user0_.dtype as dtype1_0_
from
users user0_

Get all UserWithProfile instances:

@Test
public void testQueryUsersWithProfile() {
 helper.createUsers();
 for (User user: helper.get(UserWithRole.class)) {
  System.out.println(user);
 }
}

Output:

UserWithRole{id=1, name='User-0', role='Role-0'}
UserWithRole{id=3, name='User-2', role='Role-2'}
UserWithRole{id=5, name='User-4', role='Role-4'}
UserWithRole{id=7, name='User-6', role='Role-6'}
UserWithRole{id=9, name='User-8', role='Role-8'}

SQL query issued by Hibernate:

select
userwithro0_.id as id2_0_,
 userwithro0_.name as name3_0_,
 userwithro0_.role as role4_0_
from
users userwithro0_
where
userwithro0_.dtype = 'UserWithRole'

Please, let me know if it's what you were looking for.