Hibernate query subtyped entity by natural-id

Question

I use Hibernate and want to query for entities by their natural identifier. However, it seems not to be possible to have natural ids on subtypes. I have two classes A and B where B extends A:

class A {
  long id;
}

class B extends A {
  String naturalId;
}

A is mapped in Hibernate with its own identifier. B is mapped as joined subclass. However, it is not possible to map the natural identifier of B in Hibernate because the mapping of B is a subclass mapping.

• Why is it not possible to have a natural identifier on the subclass B? Note that I don't want Hibernate to generate my database schema, I just want to have natural ids for fast cache hits.

• Is there a way/a best practice to have natural ids on subtypes for fast second level cache querying?

  • Is this still possible when natural ids might get updated (change) in rare circumstances and the cache has to be maintained in a clustered Java EE environment?

Answer 1

According to 13.3. Entity inheritance and second-level cache mapping:

As of Hibernate ORM 5.3, you can now override a base class @Cacheable or @Cache definition at subclass level.

so you might have a chance by resetting the caching annotations on B.
I never tried it so please confirm the solution with a comment.

Answer 2

1. NaturalId only make sense for base classes, because you can't retrieve a sub-class without the base class info.

   Let's say you could map map both the base class and the sub-class with a natural-id:

   class A {
     long id;
     String baseId;
   }
   
   class B extends A {
     String naturalId;
   }
   
   A a = session.bySimpleNaturalId( A.class ).load( "abc" );
   

   If the entity we retrieve if of type B, it's not clear which of the natural-id variants will be used.

2. You can't fetch a sub-class without getting the base-class info. So when you load a Sub-class from cache, the associated base-class info is retrieved as well. Therefore you could have the base class store the natural-id, or simply use the primary key for caching.

3. You can update the natural-id, although a business key should be immutable. For mutable natural-ids you need to use the mutable attribute.