Appengine JDO, how can I retrieve a child object knowing a child property and the parent key?

Using the peristance manager, how can I retrieve a child object knowing a child property and the parent key?

The Parent is defined like this:

public class User {

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

@Persistent(mappedBy = "user")
@Element(dependent = "true")
private List<Section> sections;
...

And the child is defined like this:

public class Section {

@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Long id;

@Persistent
private User user;

@Persistent
private String title;
...

Knowing the 'User' id and the 'Section' title, how can I retrieve the section?. I was trying to build a query to retrieve the section using something like this: 'where title == xxx AND user.id ¿? == xxx' but I'm not sure how to specify the user id. Is there any way to do it using queries or methods from the persistanc开发者_如何学运维e manager?

Thanks.

I finally made it with this method:

public static Section getSectionByTitle(String title, Key user_key){
    PersistenceManager pm = PMF.get().getPersistenceManager();  
    Query query = pm.newQuery("select from "+Section.class.getName()+" WHERE title == s && user == keyParam");
    query.declareParameters("String s, String k");
    query.setUnique(true);
    Section section = (Section) query.execute(title, user_key.getId());
    return section;
}

You can call this method on the query object:

q.setAncestor(ancestorKey);

Read this page for more information (Ancestor Queries).

I remember seeing something like 'where ANCESTOR = ' syntax but I cannot find any reference for it now.