Hibernate does not allow an embedded object with an int field to be null?

Question

Hibernate does not allow me to persist an object that contains an null embedded object with an integer field. For example, if I have a class called Thing that looks like this

@Entity
public class Thing {

    @Id
    public String id;

    public Part part;

}

where Part is an embeddable class that looks like this

@Embeddable
public class Part {

    public String a;    

    public int b;

}

then trying to persist a Thing object with a null Part causes Hibernate to throw an Exception. In particular, this code

Thing th = new Thing();
th.id = "thing.1";
th.part = null;
session.saveOrUpdate(th);

causes Hibernate to throw this Exception

org.hibernate.PropertyValueException: not-null property references a null or transient value: com.ace.moab.api.jobs.Thing.part

My guess is that this is happening because Part is an embedded class and so Part.a and Part.b are simply columns in the Thing database table. Since the Thing.part is null Hibernate wants to set the Part.a and Part.b column values to null for the row for thing.1. However, Part.b is an integer and Hibernate will not allow integer columns in the database to be null. This is what causes the Exception, right?

So I am looking for workarounds for this problem. I noticed making Part.b an Integer instead of an int seems to work, but for reasons I won't bore you with this is not a good option for us. Thanks!

Answer 1

You're absolutely right about what's happening. When an object of an embedded class is null, Hibernate represents this by making all its columns null in the database; unfortunately, this means Hibernate can't tell the difference between a null embeddable object and an embeddable object with all null values.

As far as I know, if you can't make the field nullable (by making it an Integer, as you guessed), the only way around this is to create a custom Hibernate type. Should be doable, but it'll be a pain to do it that way. In fact, it'd likely be worse than whatever the reason is you can't use an Integer — so what is that reason, anyway?

Answer 2

Pretty uncomfortable, yes. You can set a default value to the part field:

private Part part = new Part();

or even

private Part part = Part.NULL_PART;

(see Null object patern)

Bear in mind, that if your @Embeddable class does not contain a primitive (which has a default value), and you save an object with all nulls, the whole structure will be persisted in the database as null, because there isn't a way for hibernate to make the difference in the DB whether you don't have an object, or you have an empty one. To workaround this, if it arises, you'd have to create a dummy field (boolean is best).

Answer 3

I found another workaround and thought I'd share it. Turns out that if you make the int column nullable then Hibernate doesn't throw the PropertyValueException.

@Column(nullable = true)
public int b;

This works for us because our hibernate enabled application is the only thing that touched the database and so we can guarantee that b is null only when the part is null. Although the null part suggestion and the use of the Integer wrapper type are great suggestions, due to lots of reasons I don't really have control over, setting the column to nullable works best for us. Thanks though and I hope this helps someone else.