Hibernate - bidirectional @OneToOne

Question

I have 2 classes: User and UserPicture which have a 1:1 relationship.

public class User {
     @Id
     @GeneratedValue(strategy=GenerationType.AUTO)
     @Column(name="id", nullable = false, unique = true)
 private int id;

     private String firstname;

     private String lastname;

     @OneToOne
     @JoinColumn(name = "picture") //field named "picture" in the database
     private UserPicture userPicture;

     ..
}


public class UserPicture {

     @Id
     @GeneratedValue(strategy=GenerationType.AUTO)
     @Column(name="id", nullable = false, unique = true)
     private int id;

     private Blob image;

     @OneToOne
     @JoinColumn(name = "user")
     User user;

'user' in UserPicture will be loaded but 'userPicture' in User not - what did Im wrong?

EDIT Have to add that Im just create a UserPicture and insert them (with existing userId) - maybe I need to cascade 'user' in UserPicture?

Answer 1

In regards to your question: (because I don't have enough reputation to respond in a comment)

"All clear! Only one question more, is it possible to make userPicture in User lazy? – user1731299 Oct 24 '12 at 10:44"

Yes, it -is- possible to make it lazy fetch. However, just saying "fetchType=FetchType.Lazy" will not work. Reason being, Hibernate needs to check the joined table to see if it's a null value, or if there's a record there. Since it's a OneToOne mapping, Hibernate figures that it can save a database call by just pulling back any data that's there, since it had to check if it was null anyways. This isn't the case for x-to-many-mappings, since Hibernate knows that the 'many' means there's a list waiting on the other table... be it an empty or a populated list, it's still a list. For a single value, it has to differentiate between the actual data and a null value.

The way around this is to tell Hibernate that there will ALWAYS be a value there, and NEVER a null value. Knowing this, Hibernate can create a place-holder until it's time to fetch that data. The way you do this in annotations is to add "optional=false" to your @OneToOne annotation.

However be cautioned! There are some issues with this; including the one I'm trying to figure out now (and how I came to stumble across your question here). This optional=false makes Hibernate do a little extra validation and seems to confuse Hibernate on how it should execute inserts. So, you may want to stay away from this lazy fetching technique.

Answer 2

You have to map your classes.

public class User {
    ...
    @OneToOne (mappedBy="user")
    private UserPicture userPicture;
    ...
}

public class UserPicture {
    ...
    @OneToOne
    @JoinColumn (name="user")
    private User user;
    ...
}