Question

I have 2 classes

@Entity 
@Table(name = "user")
public class User 
@Id
@Column(name = "id")
@GeneratedValue
private int id;

@OneToMany(fetch = FetchType.LAZY, mappedBy = "user")
@Cascade({CascadeType.ALL})
private Collection<Book> userBooks;
}
@Entity 
@Table(name = "books")
public class Book
@Id
@Column(name = "id")
@GeneratedValue
private int id;

@ManyToOne
private User user;
}

here some more code:

  User user = new User();
    user.setUserName("Gleeb");
    user.setUserPassword("gleeb");

    Book book = new Book();
    book.setTitle("some book ");
    book.setbookType(1);
    user.addUserbook(book);

    userService.save(user);

i am trying to create a new user and add a new book to it.
when i save the user

session.saveOrUpdate(user);

the user is saved, the book is saved
but in the book table, the user_id is not set. and is left NULL.

why is that.

thanks.

Answer 1

You didn’t show your code, but the join table was probably not populated because you forgot to initialize the user field of the book. in a bidirectional association, the owner side is the side which doesn’t have the mappedBy attribute, and it’s this side that JPA uses to know id the association exists or not.