How can I map a parent-child relationship in JDO, where each parent has a list of children?

Question

I'm new to JDO but I'm willing to use it to to make my code portable (currently I'm using AppEngine, thats why). In the following Google Talk: http://dl.google.com/io/2009/pres/W_0415_Building_Scalable_Complex_App_Engines.pdf, Brett Slatkin showed a new and efficient pattern to retrieve parent entities using child's keys [slides 23 - 25]. In his example, he didn't have any mapping of parent-child relationships for the classes in Python. I don't know how Python works, but I'm guessing in JDO, you need to specify the parent somewhere. So for the following code, how can I map MessageIndex as a child to Message:

// Message.java
@PersistenceCapable
public class Message {
    @PrimaryKey
    @Persistent(valueStrategy=IdGeneratorStrategy.IDENTITY)
    Long id;

    @Persistent String sender;
    @Persistent Text body;
}

// MessageIndex.java
public class MessageIndex {
    @PrimaryKey
    @Persistent(valueStrategy=IdGeneratorStrategy.IDENTITY)
    Long id;

    @Persistent List<String> receivers;
}

// Query.java - Query Example
Query query = pm.newQuery("select id from MessageIndex" +
                          "where receivers == NameParam " +
                          "parameters String NameParam");
List<Key> indexes = (List<Key>) query.execute("Smith"); // List the child keys here
List<Key> keys = new List<Key>; // A place to store parent keys
for (Key k : indexes)
{
    keys.add(k.getParent()); // Here, how would getParent() know who it's parent is?
}

List <Message> messages = new List <Message>
for (Key k : keys)
{
    messages.add(pm.getObjectById(Message.class, k));
}

So, how can I map the parent-child relationship between Message and MessageIndex in this example? Did I convert the Python code correctly?

Any comments/suggestions are highly welcome!

P.S. The link for the video is here: http://www.google.com/events/io/2009/sessions/BuildingScalableComplexApps.html --> very interesting!

Thanks.

Answer 1

Never mind. I found another way of doing this. Whenever I create a Message, I'll create the MessageIndex with the same key. This way, MessageIndex is always associated with a Message because they share the same key. Now, I can simply getKey() from MessageIndex and use getObjectById() in the Message entity to get the corresponding message. I would appreciate it if anyone had any better solutions than this, but for now I'll go with this method. Thanks anyways.