Can an entity inherit from another inherited entity?

nitingupta183 · **Posted:** Sat Jun 14, 2008 3:07 am

Hi All,

I am working on a Hibernate application and using annotations to define entities in the system. I am now facing a issue in which I am not very clear if the solution I have in mind is possible or not.

I would like to know if it is possible to inherit an entity from an already inherited entity? My case is described with the help of following code snippet:

Code:

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class SuperClass{

}

@Entity
@DiscriminatorValue("test")
public class InheritedClass extends SuperClass{

}

@Entity
@DiscriminatorValue("test11")
public class InheritedClass2 extends InheritedClass{

}

I want to create the InheritedClass2. But when I save this entity, I am getting an exception.

Please let me know if it is possible to do so or not. I want to leverage the inherited class in the second class and so this extension is from the InheritedClass instead of SuperClass.

Cheers,
Nitin

Cameron McKenzie · **Posted:** Sat Jun 14, 2008 9:42 am

I actually map a very similar, simple inheritance hierarchy in one of the Hibernate3 tutorials on my website. Fortunately, I us JPA as well. Here's the scenario Java classes:

The mappings looked like this:

Ancestor without Mapping Strategy

Code:

package com.examscam.mappings; 
import javax.persistence.*;
@Entity
public class Ancestor {
  private Long id;
  private String nationality;
  @Id
  @GeneratedValue
  public Long getId() {return id;}
  public void setId(Long id) {this.id = id;}
  public String getNationality() {return nationality;}
  public void setNationality(String nationality) {
    this.nationality = nationality;
  }
}

Parent Java Class

Code:

package com.examscam.mappings;
import javax.persistence.*;
@Entity
public class  Parentextends Ancestor{
  private String lastName;
  public String getLastName() {return lastName;}
  public void setLastName(String lastName) {
        this.lastName = lastName;
  }
}

Child Java Class

Code:

package com.examscam.mappings;
import javax.persistence.*;
@Entity
public class  Parentextends Ancestor{
  private String lastName;
  public String getLastName() {return lastName;}
  public void setLastName(String lastName) {
        this.lastName = lastName;
  }
}

Then I just added the following to the ancestor class:

Code:

@Entity
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public class Ancestor {    }

Running the code in a main method looked like this:

Code:

public static void main(String args[]) {
  Ancestor a = new Ancestor();
  a.setNationality("Korean");

  Parent p = new Parent();
  p.setNationality("Jewish");  p.setLastName("Steinberg");

  Child c = new Child();
  c.setNationality("Irish"); 
  c.setLastName("McKenzie");
  c.setFirstName("Cameron");

  Session session = HibernateUtil.beginTransaction();
  session.save(a); session.save(p); session.save(c);
  HibernateUtil.commitTransaction();
}

And when it was done, here was the tables and results:

And of course, the SQL:

Code:

Hibernate:
insert into Ancestor (nationality, DTYPE) values (?, 'Ancestor')
Hibernate:
insert into Ancestor (nationality, lastName, DTYPE) values (?, ?, 'Parent')
Hibernate:
insert into Ancestor (nationality, lastName, firstName, DTYPE) values (?, ?, ?, 'Child')

nagkumar · **Posted:** Sat Jun 14, 2008 9:45 am

What u indent to do is perfectly possible. What exceptiona re u getting..

Try using InheritanceType.JOINED instead of single_table or look at the example at

http://www.oracle.com/technology/produc ... tance.html

Regards,
Nagendra