def update(id, add, delay=0):
entity = yield SomeKind.get_by_id_async(id)
foo = entity.foo
foo += add
yield ndb.sleep(delay)
entity.foo = foo
yield entity.put_async()
def callback():
transaction = ndb.get_context().transaction
yield ndb.sleep(delay)
assert ndb.get_context().transaction == transaction
raise ndb.Return(transaction)
