Object Promotion?

JamesT · **Joined:** Thu Oct 28, 2004 3:38 am **Posts:** 8

Assuming this simple mapping:

Code:

<class name="Entity"  discriminator-value="E">
    <id name="id" type="java.lang.Long">
        <generator class="identity"/>
    </id>
    <discriminator column="subclass" type="character"/>

    <!--  Numerous property definitions here -->

    <subclass name="NamedEntity" discriminator-value="N">
        <property name="name" type="java.lang.String"/>
    </subclass>
</class>

Is there any way of "promoting" an "Entity" object to a "NamedEntity" object?

I'd think it would generate an sql statement similar to the following:

Code:

update Entity set subclass='N', name=?

anthony · **Posted:** Sun Feb 13, 2005 6:02 am

do it in jdbc and don't forget to invalidate cache(s)

JamesT · **Joined:** Thu Oct 28, 2004 3:38 am **Posts:** 8

Do people use it this way often? I know I've come across a couple situations where this would be handy - unless my thinking is skewed (which is very likely ;) ).

I know it can be done in jdbc - but I thought a hibernate abstraction of this function could be helpfull in ensuring value constraints were still met (i.e. a not-null constraint on the name property of the subclass). Also, in situations where this is performed often, constant cache invalidation would cause a performance hit.

My view of how this would actually work would be something like this:

Code:

Entity entity = session.load(Entity.class, integerValue);

NamedEntity namedEntity = session.promoteToSubclass(NamedEntity.class, entity);

namedEntity.setName("Entity Name");

session.update(namedEntity);

The promoteToSubclass function would return a special proxy object that would cause the persister to create the sql described above, also the cache invalidation would be handled either at that point or when update was called (whichever is appropriate).

Thoughts?

JamesT · **Joined:** Thu Oct 28, 2004 3:38 am **Posts:** 8

Quote:

Also, in situations where this is performed often, constant cache invalidation would cause a performance hit.

I knew this wasn't true almost as soon as Isubmitted my previous post. I still think the API described above is interesting though.

anthony · **Posted:** Mon Feb 14, 2005 6:37 am

I understand that the API you described _seems_ easy... but it isn't.
Check the source and try to implement.

What you're trying to do isn't 100% related to Hibernate, try to change type in java... impossible.

Using hibernate/jdbc/refresh is a workaround, nothing else

michael · **Posted:** Mon Feb 14, 2005 7:09 am

This has also been discussed numerous times already here in the forum. Please search.

kastor · **Joined:** Tue Jan 04, 2005 4:52 pm **Posts:** 13

Hello,

You can simply set the discriminator to the same value on all your subclasses and the base class.

Then you simply get the kind you are interested in out and it ignores the properties not for the class you want.

This approach is not exactly what you wanted but will allow runtime determination of the kind of subclass to load.