OneToMany, ManyToOne why value of parent in children is null?

Question

I read similar questions, but still have problem.

Configuration info: Hibernate 3.5.1

Parent class Question:

@Entity
public class Question implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name="questionId")
    private long id;
    private String title;
    private String description;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="question")
    private Set<Vote> votes;

    public void addVote(Vote vote){
        if(votes==null)
            votes = new HashSet<Vote>();
        getVotes().add(vote);
    }

}

Child class Vote:

@Entity
public class Vote implements Serializable {
    private static final long serialVersionUID = 1L;

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

    private transient InetAddress address;

    @ManyToOne
    @JoinColumn(name="questionId")
    private Question question;

    @Enumerated(EnumType.STRING)
    private Mode mode;

// ...
}

And finally very simple test:

@Test
public void testSaveOrUpdate() {
Vote vote1 = new Vote();
    vote1.setAddress(InetAddress.getLocalHost());
    vote1.setMode(Mode.HIM);
    question = new Question();
    question.setTitle("test?");
    question.setDescription("test");
    question.addVote(vote1);

    question2 = questionDao.saveOrUpdate(question);
    assertNotNull(question2);
    Set<Vote> votes = question.getVotes();
    assertEquals(votes.size(), 1);
    for(Vote vote:votes)
        assertNotNull(vote.getQuestion());
}

test fail because vote.getQuestion() return null. When I check in DB there are null in questionId column. My question is what I should do to have reference to question in vote from votes? there is something wrong with mapping but i have no idea what.

what happens if you explicitly make a call to Vote.setQuestion(..) before persisting? — ThomasRS, Mar 15 '11 at 19:48
Get StackOverflowException because entity manager going to circle reference and generate infinity loop of inserts. — Koziołek, Mar 15 '11 at 19:50

Satadru Biswas · Accepted Answer · 2011-03-15T19:57:06.913

1

In all relationships there is an owning side (atleast in ORM). In your Many-to-One case you have made the Vote as the owning side. That means, it is the dependency of a vote to associate itself with the question. Hence question.addVote(vote1) will not work, but vote.setQuestion(question) will make everything work. The Vote is the owning side, as you have declared the JoinColumn on this side and the mappedBy field on the Question side.

Edit: And you should now persist the vote object and not the question object to have any effect.

edited Mar 15 '11 at 19:57

answered Mar 15 '11 at 19:50

Satadru Biswas

1,555
2
13
23

and then I should persist Vote not question? – Koziołek Mar 15 '11 at 19:56
1

Edited..been there, done that. Read the documentation carefully, the open source guys write in really small fonts :P – Satadru Biswas Mar 15 '11 at 19:58
The problem is not reading... problem is to know what I want to do. thx for help. – Koziołek Mar 15 '11 at 20:14
I very honest advice from my side as a engineer, read as much as you can about Design Principles and Patterns, you will know what to do when. Optimizing is the next step. May the Force be with you :D – Satadru Biswas Mar 15 '11 at 20:16

OneToMany, ManyToOne why value of parent in children is null?

1 Answers1

Linked