Inheritance in hibernate

Question

I would like to implement inheritance in Hibernate.

I created ObjectClass object:

@Entity
@Table(name = "object")
@Inheritance(strategy = InheritanceType.JOINED)
public class ObjectClass {
    private id;

}

and CodeTable object that inhertance Object class:

@Entity
@ForeignKey(name = "id")
@Table(name = "code_table")
public class CodeTable extends  ObjectClass{
    private String description;
}

in the database

object table is:

CREATE TABLE `object` (
  `id` bigint(11) NOT NULL auto_increment,
    PRIMARY KEY  (`id`),
 )

code_table table is:

-

CREATE TABLE `code_table` (
  `id` bigint(11) NOT NULL auto_increment,
  `description` varchar(45) character set latin1 default NULL,
   PRIMARY KEY  (`id`),
   KEY `FK_object` (`id`),
  CONSTRAINT `FK_object` FOREIGN KEY (`id`) REFERENCES `object` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
 ) 

I wrote the following code to retreive data from codeTable:

 @SuppressWarnings( "unchecked" )
    @Transactional( readOnly = true, propagation = Propagation.REQUIRED )
    public Collection<CodeTable> findAll() {
        Session session = getSessionFactory().getCurrentSession();

        return 
            session.createCriteria( persistentClass 
                    ).setResultTransformer( Criteria.DISTINCT_ROOT_ENTITY
                            ).list();
    }

I gets empty list although there is one record in codetable table.

When I write the following SQL in my database:

SELECT * FROM `code_table`

I get: id= 1, description = company.

What went wrong in my Hibernate definition? How can I retrieve the object?

EDITED : My hibernate.cfg.xml file looks like this:

<hibernate-configuration>
    <session-factory>
           <mapping class="com.mycompany.model.CodeTable" />
           <mapping class="com.mycompany.model.ObjectClass" />
        </session-factory>  
</hibernate-configuration>

Answer 1

Your mappings and table structure are (roughly) correct for a JOINED inheritance strategy and I cannot reproduce your problem.

I use the following mappings (which are basically the one you provided):

@Entity
@Table(name = "object")
@Inheritance(strategy = InheritanceType.JOINED)
public class ObjectClass {
    @Id @GeneratedValue 
    private Long id;

    public ObjectClass() { }

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

And

@Entity
@ForeignKey(name = "id")
@Table(name = "code_table")
public class CodeTable extends  ObjectClass{
    private String description;

    public CodeTable() { }

    public String getDescription() { return description; }
    public void setDescription(String description) { 
        this.description = description; 
    }
    @Override
    public String toString() {
    return "CodeTable [getDescription()=" + getDescription() + ", getId()="
            + getId() + "]";
    }
}

The following tables:

create table code_table (
    description varchar(255),
    id bigint not null,
    primary key (id)
)

create table object (
    id bigint not null,
    primary key (id)
)

alter table code_table 
    add constraint id 
    foreign key (id) 
    references object

And the following parent/child records:

insert into object values (1);
insert into code_table(id, description) values (1, 'foo');

And running your criteria query:

session.createCriteria(CodeTable.class)
    .setResultTransformer(Criteria.DISTINCT_ROOT_ENTITY)
    .list();

Returns:

CodeTable [getDescription()=foo, getId()=1]

Everything works as expected.

References

• JPA 1.0 Specification
  • 2.1.10 Inheritance Mapping Strategies
• Hibernate Annotations Reference Guide
  • 2.2.4. Mapping inheritance

Answer 2

How does your mapping looks like ?

Have you read this section in the Hibernate doc ? Inheritance mapping in Hibernate

As you can read in the link I provided above, your mapping is not correct. You have to let Hibernate know that the code_table class inherits from the object class, and you 'll have to let Hibernate know how this link exists in the database.