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Answer 1

+2 A:

@JoinColumn annotation is not needed on both sides. In fact, you should only put this on owning side of the relationship.

Moreover, you have describe the thing as below,

@JoinColumn(name = "project_id", updatable = false, insertable = false, nullable=false)

Its totally making no sense. This, acutally, means that you don't want to update it, neither you like to insert it, nor you want it null.

Furthermore, you are missing @Column annotation upon your Id fields, in case the name of the actual column is not id.

Adeel Ansari 2010-09-07 09:23:05

when you say owning side, do you mean parent side? that's not clear to me

Sven 2010-09-07 09:28:04

@Sven: Owning-side is the one which owns the foreign key. So, the entity for the table having foreign-key should be the owning-side. In your case it would be `Application`.

Adeel Ansari 2010-09-07 09:30:01

Thank's. I updated my post with a new exception -.-

Sven 2010-09-07 09:34:06

@Sven: Did I suggest you to remove it from the owning side? I think I said to keep it only there. :)

Adeel Ansari 2010-09-07 09:38:51

You made some good points, +1 too.

Pascal Thivent 2010-09-07 10:44:31

@Pascal: Thanks.

Adeel Ansari 2010-09-13 03:24:38

Answer 2

+3 A:

There are several things to fix in your mapping.

Your association is bidirectional, you need to use the mappedBy attribute on the "One" side (or what you'll get is two unidirectional associations) to define the field that "owns" the relation on the owning side (the side that holds the foreign key i.e. the Many side in a one-to-many).
The @JoinColumn annotation should be defined on the owning side only, not both sides.
If you are using a JPA 2.0 compliant version of Hibernate (i.e. Hibernate 3.5+), prefer the standard @OrderColumn annotation of the Hibernate specific @IndexColumn
I'm not sure to understand why you defined the @JoinColumn as totally read-only with updatable = false, insertable = false, nullable=false, this doesn't sound appropriate.

So your mappings become (assuming you're using JPA 2.0):

@Entity
public class Project {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    @OneToMany(cascade = CascadeType.ALL, mappedBy="project")
    @OrderColumn(name = "project_index")
    List<Application> applications = new ArrayList<Application>();

    // getters, setters
}

And

@Entity
public class Application {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    @ManyToOne
    @JoinColumn(name = "project_id")
    private Project project;

    // getters, setters
 }

And since your association, is bidirectional, don't forget to set both sides of the link when creating your object graph:

Project p = new Project();
Application a = new Application();
a.setProject(p);
p.getApplications().add(a);
em.persist(p);

But I would actually recommend to create link management methods in your entities to set both sides of the link, e.g.

@Entity
public class Project {
    ...
    public void addToApplications(Application app) {
        this.applications.add(app);
        app.setProject(this);
    }
    ...
}

Resources

Hibernate Core Documentation
- 1.2.6. Working bi-directional links
JPA wiki book
- Example of a OneToMany relationship and inverse ManyToOne annotations

References

JPA 1.0 specification
- Section 2.1.8.2 "Bidirectional ManyToOne / OneToMany Relationships"
- Section 9.1.24 "OneToMany Annotation"
- Section 9.1.22 "ManyToOne Annotation"
JPA 2.0 specification
- Section 11.1.39 "OrderColumn Annotation"

Pascal Thivent 2010-09-07 09:47:10

+1| Perfect. Actually, I was waiting for your post.

Adeel Ansari 2010-09-07 10:05:11

Thank you Pascal, you are kind of genius! Like BalusC as well. If I could I would shout a beer you two !

Sven 2010-09-07 12:31:15

@Adeel Thank you :) @Sven You're welcome

Pascal Thivent 2010-09-08 00:49:08

One thing: List<Application> applications = new ArrayList<Application>();....that list should not be initialized or else the list gonna be empty after query... I had hibernate to initialize it!!!

Sven 2010-09-08 09:18:10

@Sven Hmm... No, this is not true. There must be something else.

Pascal Thivent 2010-09-08 21:25:46

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