ORM supporting immutable classes

Question

Which ORM supports a domain model of immutable types?

I would like to write classes like the following (or the Scala equivalent):

class A {
  private final C c; //not mutable

  A(B b) {
     //init c
  }

  A doSomething(B b) {
     // build a new A
  }
}

The ORM has to initialized the object with the constructor. So it is possible to check invariants in the constructor. Default constructor and field/setter access to intialize is not sufficient and complicates the class' implementation.

Working with collections should be supported. If a collection is changed it should create a copy from the user perspective. (Rendering the old collection state stale. But user code can still work on (or at least read) it.) Much like the persistent data structures work.

Some words about the motivation. Suppose you have a FP-style domain object model. Now you want to persist this to a database. Who do you do that? You want to do as much as you can in a pure functional style until the evil sides effect come in. If your domain object model is not immutable you can for example not share the objects between threads. You have to copy, cache or use locks. So unless your ORM supports immutable types your constrainted in your choice of solution.

Answer 1

Hibernate has the @Immutable annotation.

And here is a guide.

Answer 2

AFAIK, there are no ORMs for .NET supporting this feature exactly as you wish. But you can take a look at BLTookit and LINQ to SQL - both provide update-by-comparison semantics and always return new objects on materialization. That's nearly what you need, but I'm not sure about collections there.

Btw, why you need this feature? I'm aware about pure functional languages & benefits of purely imutable objects (e.g. complete thread safety). But in case with ORM all the things you do with such objects are finally transformed to a sequence of SQL commands anyway. So I admit the benefits of using such objects are vaporous here.

Answer 3

Though not a real ORM, MyBatis may able to do this. I didn't try it though.

http://mybatis.org/java.html

Answer 4

You can do this with Ebean and OpenJPA (and I think you can do this with Hibernate but not sure). The ORM (Ebean/OpenJPA) will generate a default constructor (assuming the bean doesn't have one) and actually set the values of the 'final' fields. This sounds a bit odd but final fields are not always strictly final per say.