stale entries in a collection when using detached objects and JDO

Question

I have a class which has a collection. Elements in the collection also have a collection. i am fetching the class instance and detaching it. Now i am updating the collection field to a new collection and then persisting the detached object. To my surpruise the objects collection now has the old elements and the new elements. How do i fix this as i had replaced the collection with a new one?

Here is the directory bean. It has a collection of numberBeans

@PersistenceCapable(detachable="true")
@FetchGroup(name="fet_num", members={@Persistent(name="numbers")})
public class DirectoryBean {

public DirectoryBean(String nameOfCity, ArrayList<NumberBean> numbers) {
    super();
    this.nameOfCity = nameOfCity;
    this.numbers = numbers;
}

@Persistent
@PrimaryKey
private String nameOfCity;

@Persistent
@Element(dependent = "true")
private ArrayList<NumberBean> numbers;
}  

This is the numberBean. it has a collection of owners.

@PersistenceCapable(detachable = "true")
@FetchGroup(name="fet_own", members={@Persistent(name="owners")})
public class NumberBean {

public NumberBean(String areaCode, String num) {
    super();
    this.areaCode = areaCode;
    this.num = num;
    this.owners = new ArrayList<OwnerBean>();
}

@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;

@Persistent
private String areaCode;

@Persistent
private String num;
}

This is the owner bean. It has no collections

@PersistenceCapable(detachable = "true")
public class OwnerBean {

public OwnerBean(String fname, String lname) {
    super();
    this.fname = fname;
    this.lname = lname;
}

@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;

@Persistent
private String fname;

@Persistent
private String lname;
 }

Now i am fetching, detaching and setting the numebrs collection to a brand new collection. But in the result i see that directory has the old numbers and the new numbers

PersistenceManager pm = PMF.get().getPersistenceManager();
    pm.getFetchPlan().addGroup("fet_own").addGroup("fet_num");
    pm.getFetchPlan().setMaxFetchDepth(-1);

      pm.getFetchPlan().setDetachmentOptions(pm.getFetchPlan().DETACH_LOAD_FIELDS);
    pm.setDetachAllOnCommit(true);
    DirectoryBean dirBeanInstance=null;
    try{
        Key k = KeyFactory.createKey(DirectoryBean.class.getSimpleName(), "lake City");
        dirBeanInstance = pm.getObjectById(DirectoryBean.class,k);
        dirBeanInstance.getNumbers().size();
        dirBeanInstance.getNumbers().get(0).getOwners().size();

    }catch(Exception e)
    {
        e.printStackTrace();
    }finally
    {
        pm.close();
    }

ArrayList<NumberBean> newNumBeans = new ArrayList<NumberBean>();
    newNumBeans.add(new NumberBean("555", "7777777"));
    dirBeanInstance.setNumbers(newNumBeans);

    pm = PMF.get().getPersistenceManager();
    try{
        pm.makePersistent(dirBeanInstance);
    }catch(Exception e)
    {
        e.printStackTrace();
    }finally
    {
        pm.close();
    }