Using the App Engine Datastore - Beginning Java Google App Engine - page 149

you think your tests call all possible queries but your application still fails at runtime,
you'll have to edit the datastore-index.xml file and add these indexes manually.

Using Transactions
At a high level, the App Engine datastore supports transactions like most relational
databases. A transaction consists of one or more database operations that either
succeed or fail in entirety. If a transaction succeeds, then all operations are
committed to the datastore. However, if one of the operations fails, then all
operations are rolled back to their original state. An example method using
transactions is shown in Listing 7-3.

Listing 7-3. Sample transaction

import javax.jdo.Transaction;

public void createContact(Contact contact, String accountId) {

PersistenceManager pm = PMF.get().getPersistenceManager();

Transaction tx = pm.currentTransaction();

try {

// start the transaction

tx.begin();

// persist the contact

pm.makePersistent(contact);

// fetch the parent account

Account account = pm.getObjectById(Account.class, accountId);

account.incrementContacts(1);

pm.makePersistent(account);

// commit if no errors

tx.commit();

} finally {

// roll back the transactions in case of an error

if (tx.isActive()) {

tx.rollback();

}

}

}