Mapping a FunctionalJava Option<Type> with Hibernate

Question

I have a hibernate-mapped Java object, JKL, which is full of a bunch of normal hibernate-mappable fields (like strings and integers).

I'm added a new embedded field to it (which lives in the same table -- not a mapping), asdf, which is a fj.data.Option<ASDF>. I've made it an option to make it clear that this field may not actually contain anything (as opposed to having to handle null every time I access it).

How do I set up the mapping in my JKL.hbm.xml file? I'd like hibernate to automatically convert a null in the database to a none of fj.data.Option<ASDF> when it retrieves the object. It should also convert a non-null instance of ASDF to a some of fj.data.Option<ASDF>. Is there any other trickery that I have to do? Thank you.

Answer 1

I would suggest introducing FunctionalJava's Option in the accessors (getter and setter), while leaving Hibernate to handle a simple java field which is allowed to be null.

For example, for an optional Integer field:

// SQL
CREATE TABLE `JKL` (
    `JKL_ID` INTEGER PRIMARY KEY,
    `MY_FIELD` INTEGER DEFAULT NULL
)

You can map a Hibernate private field directly:

// Java
@Column(nullable = true)
private Integer myField;

You could then introduce Option at the accessor boundary:

// Java
public fj.data.Option<Integer> getMyField() {
    return fj.data.Option.fromNull(myField);
}

public void setMyField(fj.data.Option<Integer> value) {
    myField = value.toNull();
}

Does that work for your needs?

Answer 2

You can use Hibernate's custom mapping types. Documentation is here. Here is an analogous example of mapping Scala's Option to a Hibernate mapping.

Simply put, you would need to extend the org.hibernate.UserType interface. You could also create a generic-typed base class with a JKL-typed sub-type, similar to what you see in the Scala example.