[appengine-java] parent filed in child object which is obtained via query becomes null

[Art]

Dear group,

Would you mind asking the reason why, in owned one-to-many
relationship, parent filed in child object which is obtained via query
becomes null?
That is actually asked at the below posts, but I did not see the
answer.
http://groups.google.com/group/google-appengine-java/browse_frm/thread/28d9c3b99df25e43/8d9443f3f16635dc?lnk=gst&q=mappedby+child+object+null+parent+field#8d9443f3f16635dc
http://groups.google.com/group/google-appengine-java/browse_frm/thread/ae89672a9c68fcac/b17fe3358d5bf8e0?lnk=gst&q=mappedby+child+object+null+parent+field#b17fe3358d5bf8e0

By using the Book class and Chapter class from
http://groups.google.com/group/google-appengine-java/browse_frm/thread/28d9c3b99df25e43/8d9443f3f16635dc?lnk=gst&q=mappedby+child+object+null+parent+field#8d9443f3f16635dc:

public void testRelation() throws Exception {
        Book b = new Book();
        b.title = "JDO 4eva";

        Chapter c1 = new Chapter();
        c1.title = "Intro";
        c1.numPages = 10;

        b.chapters.add( c1);

        Chapter c2 = new Chapter();
        c2.title = "Configuration";
        c2.numPages = 9;
        b.chapters.add( c2);

        pm = pmf.getPersistenceManager();
        pm.setDetachAllOnCommit( true);
        pm.currentTransaction().begin();
        try {
                pm.makePersistent( b);
                pm.currentTransaction().commit();

                assertEquals( b, c1.book);
                assertEquals( b, c2.book);

        } finally {
                if (pm.currentTransaction().isActive()) {
                        pm.currentTransaction().rollback();
                }
                pm.close();
        }

        // Confirm on child field in parent object
        pm = pmf.getPersistenceManager();
        Query query = pm.newQuery( Book.class);
        List<Book> bookList = (List<Book>)query.execute();
        // Sanity check on book
        assertEquals( 1, bookList.size());
        Book bookEntityGroup = bookList.get( 0);
        assertEquals( b.title, bookEntityGroup.title);
        // Sanity check on chapters field
        assertEquals( 2, bookEntityGroup.chapters.size());
        for( Chapter chapter : bookEntityGroup.chapters) {
                assertTrue(
                                chapter.title.equals( c1.title) ? true :
chapter.title.equals( c2.title) ? true : false
                                );
        } // for
        pm.close();

        // Confirm on parent field in child object
        pm = pmf.getPersistenceManager();
        query = pm.newQuery( Chapter.class);
        List<Chapter> chapterList = (List<Chapter>)query.execute();
        assertEquals( 2, chapterList.size());
        for( Chapter chapter : chapterList) {
                assertTrue(
                                chapter.title.equals( c1.title) ? true :
chapter.title.equals( c2.title) ? true : false
                                );
                assertNotNull( chapter.book);   // This fails
        } // for
        query.close( chapterList);
        pm.close();
}

Warm regards,
Art

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