Question

If I have one level of inheritance, everything is persisted as expected in App Engine:

Worker.java

import javax.jdo.annotations.IdGeneratorStrategy;
import javax.jdo.annotations.Inheritance;
import javax.jdo.annotations.InheritanceStrategy;
import javax.jdo.annotations.PersistenceCapable;
import javax.jdo.annotations.Persistent;
import javax.jdo.annotations.PrimaryKey;

@PersistenceCapable
@Inheritance(strategy = InheritanceStrategy.SUBCLASS_TABLE)
public abstract class Worker {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent
    private String department;
}

Employee.java

// ... imports ...

@PersistenceCapable
public class Employee extends Worker {
    @Persistent
    private int salary;

}

Intern.java

import java.util.Date;
// ... imports ...

@PersistenceCapable
public class Intern extends Worker {
    @Persistent
    private Date internshipEndDate;
}

However, if I add one additional layer of inheritance, the fields in the highest level subclass are not persisted:

Human.java

@PersistenceCapable
public abstract class Human {

    @Persistent
    private String name;
}

Worker.java

@PersistenceCapable
@Inheritance(strategy = InheritanceStrategy.SUBCLASS_TABLE)
public abstract class Worker extends Human {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent
    private String department;
}

Why is it ok to extend an abstract class but not ok to have that abstract class extend a higher level class?

Answer 1

And your Primary key is now not in the root class of the (persistable) inheritance tree, hence invalid by the JDO and JPA specs.