Persist new Suclass Object

jordi · **Joined:** Wed Feb 18, 2009 4:53 am **Posts:** 4

I've got a problem using Spring 2.5.5, Hibernate 3.2.4.sp1 and Postgres.

I'm using a base class (BaseClass) with common data and a set of subclasses with more information, using a table-per-class schema sharing the same id. That works.

There is now the need to specialize one of the subclasses (SubClass). I've created a new subclass of it (SpecializedSubClass).

Code:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@SequenceGenerator(name = "SEQ_GEN", sequenceName = "baseclass_sequence", allocationSize = 1)
public class BaseClass  {
   @Id
   @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SEQ_GEN")
}   protected Long id;

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public class 
SubClass extends BaseClass {
   @Column
   private String column1;
}

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public class 
SpecializedSubClass extends SubClass {
   @Column
   private String column2;
}

When I create for the existing objects in the database the new objects that works as expected. But when I try to create a new SpecializedSubClass for an existing SubClass in the runtime I get an error:

Code:

HibernateSystemException: a different object with the same identifier value was already associated with the session

If I evict the SubClass object then I get the following exception:

Code:

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.orm.hibernate3.HibernateOptimisticLockingFailureException: Object of class [SpecializedSubClass] with identifier [945]: optimistic locking failed; nested exception is org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [SpecializedSubClass#945]

To do it I've created a new SpecializedSubClass object copying all the fields of SubClass and assigning it the same id. Is that a wrong way of doing it? Does anybody know how to do it?

Thanks in advance,
Jordi

jordi · **Joined:** Wed Feb 18, 2009 4:53 am **Posts:** 4

Any Ideas or hints are appreciated!

mmerder · **Posted:** Wed Feb 18, 2009 9:56 am

Well, your approach is not very "Java-like". You cannot instantiate for example a vehicle-object and later instantiate a subclass of Vehicle saying this subclass-object also is the same object as your vehicle.

There are two alternatives to think about:
1. always create a specialized subclass, as (by polymorphism definition) a "subclass is a class", leaving the extra property null.
2. Use associations instead of inheritance, if objects temporarily have other properties.

jordi · **Joined:** Wed Feb 18, 2009 4:53 am **Posts:** 4

To be exact that's like "I have a car, and now I have a tunned car". A tunned car has a lot of properties that a normal car doesn't, and I would like to mantain it separated, as any existing car can be transformed every time to a tunned car.
The idea was to load a car, look if it's an instance of tunned car, and change the logic according to that.
I can always create the specialized subclass and have an extra property "isTunned", but I thought it could be better deciding it from the instance type.

Anyway, how would you do it with associations?

mmerder · **Posted:** Wed Feb 18, 2009 10:42 am

You could create a class TunedProperties (the name could be better ;-)) where you include these special properties. To car you would add an attribute of that class.

So a car is only tuned if this attribute tunedProperties is not null.

An alternative in that case would be to use TunedProperties as @Embeddable, so that these extra properties are stored in the car table. hibernate would set tunedProperties to null then, if any if the properties of that embedded object are null.

jordi · **Joined:** Wed Feb 18, 2009 4:53 am **Posts:** 4

That's exactly what I needed, Thanks a lot!