Swap two elements in a list using JDO on Google Appengine

Question

I have a TestEntity that has an ArrayList of ChildEntities. They are joined in a owned relationship. I want to get the Entity from the datastore, update an int field in the child entity, and then swap the position of the two child entities. However, as soon as I swap the entities, it appears as though the updates to the int field are erased. Am I not allowed to store a JDO persisted object in a temp variable to do the swap? Here is my test code, followed by the definitions of the Entities themselves. When I step through the code in the debugger, my ChildEntity gets overwritten or reset or whatever as soon as I copy the 2nd value into position 0 in the list. I'm afraid there's something about how JDO works that I am missing.

@Test
public void testSwap() {
    PersistenceManager pm = PMF.get().getPersistenceManager();

    TestEntity te = new TestEntity();
    pm.makePersistent(te);
    te.getChildEntities().add(new TestChildEntity("a"));
    te.getChildEntities().add(new TestChildEntity("b"));
    long id = te.getId();
    pm.close();
    pm = PMF.get().getPersistenceManager();
    te = pm.getObjectById(TestEntity.class, id);

    List<TestChildEntity> children = te.getChildEntities();
    for (TestChildEntity tce : children) {
     tce.setFoo(3);
    }

    TestChildEntity temp = children.get(0);
    children.set(0, children.get(1));   

        // after the above line executes, the object referred to by temp is overwritten/reset when i watch in debugger.

    children.set(1, temp);
    assertEquals(3, children.get(0).getFoo());  // these asserts will both fail.
    assertEquals(3, children.get(1).getFoo());

}

@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class TestEntity {
        @PrimaryKey
        @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
        private Long id;

        @Persistent(mappedBy = "parent")
        private List<TestChildEntity> childEntities;

        public List<TestChildEntity> getChildEntities() {
                return childEntities;
        }

        public Long getId() {
                return id;
        }

        public static void demonstrateProblemWithRemove() {
                PersistenceManager pm = PMF.get().getPersistenceManager();
                TestEntity te = new TestEntity();
                pm.makePersistent(te);
                te.getChildEntities().add(new TestChildEntity("a"));
                te.getChildEntities().remove(0);
                pm.close();
        }

}

@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class TestChildEntity {
        @PrimaryKey
        @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
        @Extension(vendorName = "datanucleus", key = "gae.encoded-pk", value
= "true")
        private String encodedKey;

        @Persistent
        @Extension(vendorName = "datanucleus", key = "gae.pk-name", value =
"true")
        private String keyName;

        @Persistent
        private int foo;

        @Persistent
        private TestEntity parent;

        public TestChildEntity(String k) {
                this.keyName = k;
                this.foo = 1;
        }

        public int getFoo() {
                return foo;
        }

        public void setFoo(int foo) {
                this.foo = foo;
        }

Answer 1

Try adding defaultFetchGroup = "true" to @Persistent annotation above TestEntity.childEntities

By default, childEntities aren't fetched from datastore (it is JDO behavior, as far as I know, nothing specific to AppEngine). In your case, I suppose, they are even not written because you call makePersistent before you add children.