Question

I am not sure if I am understanding the google documentation, I am wondering if someone else can check my understanding.

Is the following code guaranteed to only ever do both of the following:

• update to the bank account balance
• store a record of the transaction.

Below is what I understand is correct:

public void add(String account, Double value, String description, Date date) throws EntityNotFoundException {
    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
    int retries = 3;
    while (true) {
        Transaction txn = datastore.beginTransaction();
        try {

            // Update the bank balance
            Key key = KeyFactory.createKey("Account", account);
            Entity e = datastore.get(key);
            Double balance = (Double) e.getProperty("balance");
            balance += value;
            e.setProperty("balance", value);
            datastore.put(e);

            // Record transaction details
            Entity d = new Entity("Transaction", key);
            d.setProperty("account_key", key);
            d.setProperty("date", date);
            d.setProperty("value", value);
            d.setProperty("description", description);

            txn.commit();
            break;
        } catch (ConcurrentModificationException e) {
            if (retries == 0) throw e;
            retries--;
        } finally {
            if (txn.isActive()) txn.rollback();
        }
    }
    }
}

Answer 1

That’s correct. It’s not necessary to include the key of the account in the entity, though – the entity you’re creating is a child of the account in question.