JDO query using the object ID

I have a User object which has a collection of Transaction objects und开发者_如何学Goer it.

I store the object ID for my User object and now need a query to sum the transactions under the User object:

    @Persistable
    public class User {
      private String username;
      private Collection<Transaction> transactions
      //...getter/setters...
    }

    @Persistable
    public class Transaction {
      private int txnAmount;
      //...getter/setter...
    }

Given the User's object ID, I want a sum of all Transaction.txnAmount's in the User.transactions collection.

My shot at this was:

    Query query = pm.newQuery(User.class);
    query.setFilter("JDOHelper.getObjectId(this) == :userIDParam");
    query.setResult("sum(transactions.credits)");
    query.execute(userID);

I have verified that userID is indeed a Database Identifier object as it should be (I can use it to query the User object by ID using it). But I get an exception in the query.execute() method:

javax.jdo.JDOException: Invocation of method "JDOHelper.getObjectId" is on null but this is not currently supported by SODA queries

Using DB4O as the datastore, DataNucleus 2.2.1 as the JDO implementation

I have a strong feeling that this issue is related to the bug report found below:

http://www.datanucleus.org/servlet/jira/browse/NUCDBFO-48