JPA: Implementing Model Hierarchy - @MappedSuperclass vs. @Inheritance

Question

I am using Play Framework 1.2.4 with PostgreSQL and JPA. I would like to have a Model hierarchy and see that there are some alternatives to doing this.

I have a base class (which is abstract) and two concrete classes extending this base class. I don't want to persist this base class while I want to have concrete classes. In the base class, I have another Model classes as properties, in other words, I have @ManyToOne relationships in my base class.

My question is what is the best way of implementing this? Using @MappedSuperclass or @Inheritance with TABLE_PER_CLASS strategy? I am a bit confused as they seem virtually equivalent.

I also have some concerns about querying and performance issues that I might face in future.

Answer 1

MappedSuperClass must be used to inherit properties, associations, and methods.

Entity inheritance must be used when you have an entity, and several sub-entities.

You can tell if you need one or the other by answering this questions: is there some other entity in the model which could have an association with the base class?

If yes, then the base class is in fact an entity, and you should use entity inheritance. If no, then the base class is in fact a class that contains attributes and methods that are common to several unrelated entities, and you should use a mapped superclass.

For example:

• You can have several kinds of messages: SMS messages, email messages, or phone messages. And a person has a list of messages. You can also have a reminder linked to a message, regardless of the kind of message. In this case, Message is clearly an entity, and entity inheritance must be used.
• All your domain objects could have a creation date, modification date and ID, and you could thus make them inherit from a base AbstractDomainObject class. But no entity will ever have an association to an AbstractDomainObject. It will always be an association to a more specific entity: Customer, Company, whatever. In this case, it makes sense to use a MappedSuperClass.

Answer 2

@MappedSupperclass is different than the @Inheritance annotation.

@MappedSuperclass tells the JPA provider to include the base class persistent properties as if they were declared by the child class extending the superclass annotated with @MappedSuperclass.

However, the inheritance is only visible in the OOP world, since, from a database perspective, there's no indication of the base class. Only the child class entity will have an associated mapped table.

The @Inheritance annotation is meant to materialize the OOP inheritance model in the database table structure. More, you can query a base class annotated with @Inheritance but you can't do that for a base class annotated with @MappedSuperclass.

Now, the reason why you'd want to use the @Inheritance JPA annotation is to implement behavior-driven patterns like the Strategy Pattern.

On the other hand, @MappedSuperclass is just a way to reuse both basic properties, associations, and even the entity @Id using a common base class. Nevertheless, you can achieve almost the same goal using an @Embeddable type. The only major difference is that you can't reuse an @Id definition with @Embeddable, but you can do it with @MappedSuperclass.