Hibernate外键语法

Question

I am trying to learn hibernate. I have a movie table with a foreign key to a genre table. Each movie is assigned to a single genre. Each genre may be assigned to many movies.

Here are the table definitions:

CREATE TABLE `movie` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(64) NOT NULL,
  `genre_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_genre` (`genre_id`),
  CONSTRAINT `fk_genre` FOREIGN KEY (`genre_id`) REFERENCES `genre` (`id`) ON UPDATE CASCADE,

)

CREATE TABLE IF NOT EXISTS `genre` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) 

For code I have

@Entity
public class Movie implements java.io.Serializable {

    private static final long serialVersionUID = 1;
    private Integer id;
    private String title;
    private Genre genre;
    ...

    @ManyToOne
    public Genre getGenre() {
        return genre;
    }

Also

@Entity
public class Genre implements java.io.Serializable {

    private static final long serialVersionUID = 1;
    private Integer id;
    private String name;

    @Id
    @GeneratedValue
    @Column(name = "id")
    public Integer getId() {
       return this.id;
    }

Then the select generated by hibernate looks like

select
    movie0_.id as column1_4_,
    movie0_.genre_id as genre11_4_,
    movie0_.title as title4_
from
    Movie movie0_

And this not right as there's no reference to the genre table. The correct query should have a join with the genre table. More like

select
    movie0_.id as column1_4_,
    genre.name as genre11_4_,
    movie0_.title as title4_
from
    Movie movie0_, Genre genre
where
    movie0_.genre_id = genre.id;

I'm a little bit of a loss as to what I'm doing wrong. Should the many to one annotation be in the Genre class instead of the Movie class? Or do you see anything else that I'm doing wrong?

Based on the advise below, Movie now has

@Override
public String toString() {
    StringBuilder sb = new StringBuilder();
    sb.append(id).append(" ");
    sb.append(title).append(" ");

    this.getGenre(); //new

    sb.append(genre.getName());
    return sb.toString();
}

@ManyToOne(fetch=FetchType.EAGER) //new
public Genre getGenre() {
    return genre;
}

And the way I'm loading Movie is through

public static void main(String[] args) {
    SessionFactory sf = HibernateUtil.getSessionFactory();
    Session session = sf.openSession();
    List<Movie> movies = session.createQuery("from Movie").list();
    for (Movie movie : movies) {
        System.out.println(movie.toString());
    }
    session.close();
}

What I'm seeing is that even though I have the eager load and I'm explicitly saying getGenre in toString, no query is generated and I'm just getting a null back.

Answer 1

When you use HQL syntax (eg createQuery("from Movie") ), then Hibernate/JPA will only fetch the Genre entity when you call getGenre() on your Movie object. This is called "lazy fetching". When the method is called, Hibernate will issue another query to fetch the Genre .

Note that HQL queries ignore the FetchType on your annotations - HQL is used to tell Hibernate exactly what to do, rather than using the hints in the annotations.

To make it fetch the Genre in the same query as the Movie , you need to tell it to:

createQuery("from Movie m join fetch m.genre")

Answer 2

try this:

Moive side:

@ManyToOne
@JoinColumn(name = "genre_id")
public Genre getGenre() {..}

and on the other side (genre):

@OneToMany(mappedBy="genre")
List/Set getMovies(){..}

then you can from a movie object movie.getGenre() get Genre.