Father f = new Father();
f.foo = "foo";
ofy().save().entity(f).now();
Child ch = new Child();
ch.father = Ref.create(Key.create(f));
ch.bar = "bar";
Key<Child> kch = ofy().save().entity(ch).now();
LoadResult<Child> fetchedRef = ofy().load().key(kch);
Child fetched = fetchedRef.now();
assert fetched.bar.equals(ch.bar);
assert fetched.father.get().id.equals(f.id);
assert fetched.father.get().foo.equals(f.foo);
}