Question

I have the following database structure:

CREATE TABLE `author` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
`email` varchar(255) NOT NULL,
`password` varchar(255) NOT NULL,
 PRIMARY KEY (`id`));

CREATE TABLE `message` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`title` varchar(500) NOT NULL,
`text` varchar(50000) NOT NULL,
`author_id` int(10) unsigned DEFAULT NULL,
`creation_date` datetime NOT NULL,
`last_update_date` datetime NOT NULL,
 PRIMARY KEY (`id`),
 KEY `author_id_fk` (`author_id`),
 CONSTRAINT `message_ibfk_1` FOREIGN KEY (`author_id`) REFERENCES `author` (`id`));

CREATE TABLE `comment` (
`id` int(10) unsigned NOT NULL,
`post_id` int(10) unsigned NOT NULL,
 KEY `message_id_fk` (`id`),
 KEY `post_id_fk` (`post_id`),
 CONSTRAINT `comment_ibfk_1` FOREIGN KEY (`id`) REFERENCES `message` (`id`),
 CONSTRAINT `comment_ibfk_2` FOREIGN KEY (`post_id`) REFERENCES `post` (`id`));

CREATE TABLE `post` (
`id` int(10) unsigned NOT NULL,
 KEY `message_id_fk` (`id`),
 CONSTRAINT `post_ibfk_1` FOREIGN KEY (`id`) REFERENCES `message` (`id`) ON DELETE CASCADE);

And the following mapping with hibernate(3.5.4-Final):

@Entity
@Table(name = "author")
public class Author {
    private Long id = 0L;
    private String name;
    private String email;
    private String password;
    private Set<Post> posts;
    private Set<Comment> comments;

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

    public void setId(Long id) {
        this.id = id;
    }

    @Column(name = "name")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Column(name = "email")
    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @Column(name = "password")
    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @OneToMany(mappedBy = "author")
    public Set<Post> getPosts() {
        return posts;
    }

    public void setPosts(Set<Post> posts) {
        this.posts = posts;
    }

    @OneToMany(mappedBy = "author")
    public Set<Comment> getComments() {
        return comments;
    }

    public void setComments(Set<Comment> comments) {
        this.comments = comments;
    }
}

@MappedSuperclass
@Table(name = "message")
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class Message implements Serializable {
    private Long id;
    private String title;
    private String text;
    private Author author;
    private Date creationDate;
    private Date lastUpdateDate;

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

    public void setId(Long id) {
        this.id = id;
    }

    @Column(name = "title")
    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    @Column(name = "text")
    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    @ManyToOne
    @JoinColumn(name = "author_id")
    public Author getAuthor() {
        return author;
    }

    public void setAuthor(Author author) {
        this.author = author;
    }

    @Column(name = "creation_date")
    public Date getCreationDate() {
        return creationDate;
    }

    public void setCreationDate(Date creationDate) {
        this.creationDate = creationDate;
    }

    @Column(name = "last_update_date")
    public Date getLastUpdateDate() {
        return lastUpdateDate;
    }

    public void setLastUpdateDate(Date lastUpdateDate) {
        this.lastUpdateDate = lastUpdateDate;
    }
}

@Entity
@Table(name = "comment")
@PrimaryKeyJoinColumn(name="id")
public class Comment extends Message {
    private static final long serialVersionUID = 1L;
    private Post post;

    @ManyToOne
    @JoinColumn(name = "post_id")
    public Post getPost() {
        return post;
    }

    public void setPost(Post post) {
        this.post = post;
    }

}

@Entity
@Table(name = "post")
@PrimaryKeyJoinColumn(name="id")
public class Post extends Message {
    private static final long serialVersionUID = 1L;
    private Set<Comment> comments;

    @OneToMany(mappedBy = "post")
    public Set<Comment> getComments() {
        return comments;
    }

    public void setComments(Set<Comment> comments) {
        this.comments = comments;
    }
}

The main idea is that Comment and Post are inherited from Message and I would like both of them to have bidirectional relation. But when I run the following code:

Session session = HibernateUtil.getSessionFactory().getCurrentSession();
Transaction tx = session.beginTransaction();
Author author = new Author();
author.setName("mike");
author.setPassword("123");
author.setEmail("[e-mail]");

Post post = new Post();
post.setAuthor(author);
post.setCreationDate(new Date());
post.setLastUpdateDate(new Date());
post.setText("Text");
post.setTitle("Title");

Long authorId = (Long)session.save(author);
Long postId = (Long)session.save(post);

tx.commit();

I get the following error:

ERROR JDBCExceptionReporter:101 - Unknown column 'author_id' in 'field list'
Exception in thread "main" org.hibernate.exception.SQLGrammarException: could not insert: [org.blogsample.mappingbeans.Post]
    at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:92)
    at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)
    at org.hibernate.id.insert.AbstractReturningDelegate.performInsert(AbstractReturningDelegate.java:64)

Update
As @JB Nizet mentioned before I changed @MappedSuperclass to @Entity, after that I’ve got another error mappedBy reference an unknown target entity property: org.blogsample.mappingbeans.Comment.author, this was solved by changing db structure(removed author_id from message table, added it to each of comment, post and created foreign keys for this column) and moving author(and getter/setter with mapping) to Comment, Post classes.

Answer 1

Your Message class shouldn’t be annotated with @MappedSuperclass, but with @Entity. @MappedSuperclass means that entities extending this class inherit columns and associations from the super class, but these columns and associations go in the table of the subclass. author_id is not in the comment table or in the post table. It’s in the message table.

Moreover @Table can only be used with an entity. Not with a mapped superclass, which is only used to inherit fields and associations, but is not mapped to its own table like an entity is.