Null values not allowed in column - legacy database

I've got a legacy DB2 database which I can't change. It can't save null-values so all foreign keys which are empty are set to zero (0).

While reading this is no problem, I just set the not found action to ignore and it returns me null instead of the child-object.

But I have a problem the other way around. When I want to save a null-object it should set the foreign key in my table to 0, but it doesn't know how to do that and throws me an "null value not allowed"-exception.

@OneToOne()
@NotFound(action = NotFoundAction.IGNORE)
@JoinColumn(name = "robj02", referencedColumnName = "objnr")
private Objekt robj02;

Is there a way to define a default value or something similar? Does anyone have an idea to help me?

You can correct your mapping in the following way:

@Entity
public class YourEntity
{
   @Column(name = "robj02")
   private Long objektId;
   @OneToOne
   @NotFound(action = NotFoundAction.IGNORE)
   @JoinColumn(name = "robj02", referencedColumnName = "objnr", insertable = false, updatable = false)
   private Objekt robj02;
   
   // getters/setters
}

and then just persist it in the following way:

YourEntity entity = new YourEntity();
entity.setObjektId(0L);
// ...
entityManager.persist(entity);

Dirty hack but something like

void setRobj02(Objeckt robj02) {
    this.robj02 = (robj02 != null)
        ? robj02
        : Objekt.ZERO // you should create it first
}