Child's id not present after cascade persisting the parent

Question

I have a User (parent) and a Home (child) entities related following a unidirectional one-to-many relationship.

My problem is that, when adding a new Home to a User, the newly created and persisted Home doesn't have the id. Is this normal? Do I need to manually persist the child if I want the id?

These are my entities:

@Entity
@Table(name = "user")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;

    @NotNull
    @Column(name = "firstName")
    private String firstName;

    @NotNull
    @Column(name = "lastName")
    private String lastName;

    @NotNull
    @Column(name = "email")
    private String email;

    @OneToMany(targetEntity = Home.class, fetch = FetchType.EAGER, cascade = {CascadeType.ALL}, orphanRemoval = true)
    @JoinColumn(name = "userId", referencedColumnName = "id", nullable = false)
    private List<Home> homes;


    public User() {
    }

    public void addHome(Home home) {
        homes.add(home);
    }
}



@Entity
@Table(name = "home")
public class Home implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;

    @NotNull
    @Column(name = "isActive")
    private Boolean isActive;


    public Home() {
    }

}

And the code to update the parent:

Home home = HomeParser.parse(homeDTO);
User user = userService.findById(userId);
user.addHome(home);
userService.update(user); // delegate call to getEntityManager().merge(user);

At this point I assumed I would have home to have the id that it's just been given when persisted to db, but it doesn't.

I already tried adding insertable = false to the Home's id's @Column, as pointed here, but it doesn't work either.

Answer 1

EntityManager.merge method implementation delegates to Hibernate Session.merge:

Copy the state of the given object onto the persistent object with the same identifier. If there is no persistent instance currently associated with the session, it will be loaded. Return the persistent instance. If the given instance is unsaved, save a copy of and return it as a newly persistent instance. The given instance does not become associated with the session. This operation cascades to associated instances if the association is mapped with cascade="merge".

Notice the part in bold. Basically, the merge operation will be cascaded to the Home instance you created, but Hibernate will create a copy of that instance, merge your instance into the copy, save the copy and replace your instance with the copy in the User.homes collection.

After this, User.homes collection should contain the copy of the Home instance with a properly initialized id.