self join in google app engine (java)

Question

Hi

I have modified the guestbook example shipped with google app engine (java) to include a parent-child relationship using some sort of a 'self join'.

Now my greeting.java file looks like this

    package guestbook;

import java.util.Date;
import java.util.List;

import javax.jdo.annotations.IdGeneratorStrategy;
import javax.jdo.annotations.IdentityType;
import javax.jdo.annotations.PersistenceCapable;
import javax.jdo.annotations.Persistent;
import javax.jdo.annotations.PrimaryKey;

import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.users.User;

@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class Greeting {
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key id;

    @Persistent
    private User author;

    @Persistent
    private String content;

    @Persistent
    private Date date;

    @Persistent
    private Greeting parent;

    @Persistent(mappedBy="parent")
    private List<Greeting> children;

    public Greeting getParent() {
     return parent;
    }

    public void setParent(Greeting parent) {
     this.parent = parent;
    }

    public List<Greeting> getChildren() {
     return children;
    }

    public void setChildren(List<Greeting> children) {
     this.children = children;
    }

    public Greeting(User author, String content, Date date) {
        this.author = author;
        this.content = content;
        this.date = date;
    }

    public Key getId() {
        return id;
    }

    public User getAuthor() {
        return author;
    }

    public String getContent() {
        return content;
    }

    public Date getDate() {
        return date;
    }

    public void setAuthor(User author) {
        this.author = author;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public void setDate(Date date) {
        this.date = date;
    }
}

Notice the addition of the parent and child fields, and changing the primary key type to com.google.appengine.api.datastore.Key (as shown in http://code.google.com/appengine/docs/java/datastore/relationships.html#Owned%5FOne%5Fto%5FMany%5FRelationships)

Now it wont save the data to the datastore. I fail to understand why. I have tried deleting the local datastore and index file(as said somewhere on the web) but it wont work. no exceptions, nothing.

Can someone look into it and please help

Answer 1

Turns out this is a known issue in google app engine

http://code.google.com/p/datanucleus-appengine/issues/detail?id=119

Now i will need to change my data model to work with app engine