Question

I recently added Text field to one of my Entities that cannot be null. I’d like to set a default value for it, so that all of the Entities that I stored before I added the field will be populated with an empty string. Is this possible with JDO?

Answer 1

Yes, though not as trivially as you were probably expecting.

Limitations

1. Will time out if it
   takes more then 30 seconds,
   unless you run it as a task, in which
   case it will time out if it takes
   more then 10 minutes.
2. There’s no smarter way to get only
   the entities that need updated since
   you can’t query on a property that
   doesn’t exist.

Workarounds

1. You’ll want to look into the
   appengine-mapreduce project to
   get an implementation that can
   complete with more then 10 minutes
   wall-clock time.
2. None known.

Code

void updateNullBarField() {
  final Text DEFAULT_BAR = new Text("bar");

  PersistenceManagerFactory pmfInstance = JDOHelper
    .getPersistenceManagerFactory("transactions-optional");
  PersistenceManager pm = pmfInstance.getPersistenceManager();
  Query query = pm.newQuery(Foo.class);
  @SuppressWarnings("unchecked")
  Collection<Foo> foos = pm.detachCopyAll((List<Foo>) query.execute());

  for (Foo foo : foos) {
    if (foo.bar == null) {
      foo.bar = DEFAULT_BAR;
      pm.detachCopy(pm.makePersistent(foo));
    }
  }
}