Question

I am trying to make a simple example of Hibernate. I have two entities: User and Note. They have relation a one to many (one user can have a lot of notes). Please help me to correctly display these relationships in a database using annotations.But I don’t want to create third table for implementation of the relation. I need to have only two tables:

Here are my classes:

User.java:

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

@Id
@GeneratedValue
@Column(name = "id")
private Long id;

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

@OneToMany(cascade = CascadeType.ALL, mappedBy="user") //Is it right value for  mappedBy-parameter?
private List<Note> notes = new ArrayList<Note>();

    // getters and setters

Note.java:

@Entity
@Table(name = "note")
public class Note {

@Id
@GeneratedValue
@Column(name = "id")
private Long id;

@Column(name = "content")
private String content;

@ManyToOne
private User user;

    // getters and setters

Main.java:

public static void main(String[] args) {

    Session session = HibernateUtil.getSessionFactory().openSession();
    Transaction transaction = null;
    try {
        transaction = session.beginTransaction();

        List<Note> notes = new ArrayList<Note>();
        Note note1 = new Note();
        note1.setContent("my first note");
        Note note2 = new Note();
        note2.setContent("my second note");
        notes.add(note1);       
        notes.add(note2);   
        User user = new User();
        user.setName("Andrei");
        user.setNotes(notes);
        session.save(user);

        transaction.commit();
    } catch (HibernateException e) {
        transaction.rollback();
        e.printStackTrace();
    } finally {
        session.close();
    }

}

hibernate.cfg.xml:

<property name="dialect">org.hibernate.dialect.MySQLInnoDBDialect</property>
    <property name="connection.pool_size">1</property>
    <property name="current_session_context_class">thread</property>
    <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
    <property name="show_sql">true</property>        
    <property name="hbm2ddl.auto">create-drop</property>
    <mapping class="com.vaannila.blog.User" />
    <mapping class="com.vaannila.blog.Note" />

After executing this code in my database Hibernate has created and has filled two tables:

But I encounter a problem: the field user_id value is null in note table.Although it must be equal to the user id (in this case 1).

What do I need to add to annotations for to solve this problem and this example to work correctly? But without creating additional tables.

I would really appreciate any help!

Answer 1

You must set the User inside each note as you have defined a bidirectional relationship. Instead of letting clients pass in a list of notes directly, create User.addNote and have it set the relationship correctly.

class User {
    ...
    public void addNote(Note note) {
        note.user = this;
        notes.add(note);
    }
}

Your test code thus becomes

Note note1 = new Note();
note1.setContent("my first note");
Note note2 = new Note();
note2.setContent("my second note");
User user = new User();
user.setName("Andrei");
user.addNote(note1);
user.addNote(note2);
session.save(user);

You can further improve this by adding the basic fields to the constructors of your objects simplifying the above to

User user = new User("Andrei");
user.addNote(new Note("my first note"));
user.addNote(new Note("my second note"));
session.save(user);