Hibernate Inheritance Mapping and Attribute Override

Question

The system I'm working on has a domain object named Order which inherits from AbstractPersistentObject. Now I need to add another domain object named ExternalOrder which has some of the properties of Order but not all. I would like these two objects to be treated polimorphically in certain places. Hence I'm thinking of implementing inheritance mapping.

I've created an AbstractOrder which now extends AbstractPersistentObject and moved the common properties to AbstractOrder. Order and ExternalOrder now extends AbstractOrder.

Since the Order table already has lots of data in the database, I would prefer not to make too many changes to the schema.

1. If I omit InheritanceType.SINGLE_TABLE, which inheritance strategy would be better for me? I should mention I've to use OnetoMany join in at least one domain Object. The domain object would refer to AbstractOrder and Hibernate would decide at runtime the concrete subclass for this AbstractOrder.

2. AbstractPersistentObject has @Id and @GeneratedValue(strategy=GenerationType.IDENTITY) for the property id. Is there a way to override this when I implement inheritance? As I understand, GenerationType.IDENTITY is not going to work for certain inheritance choices and I'm not sure how to override this. I've looked into @AttributeOverride but I think it's only useful when you want to override certain @Column values.

Answer 1

1) You don't really have too many options here. Two other inheritance mapping strategies are table per class which won't work for you (as it doesn't support IDENTITY) and joined subclasses which will require you to split your "Orders" table into two (AbstractOrder and Order) plus add another table for ExternalOrder.

2) You can't override attributes on id within inheritance hierarchy. That's just as well, though, table-per-class strategy is definitely not ideal.