class MyIdGenerator extends AbstractIdGenerator
{
    public function generate(EntityManager $em, $entity)
    {
        $stmt = $em->getConnection()->prepare(sprintf('SELECT %s.nextval id FROM DUAL', $entity->getGeneratorName()));
        $stmt->execute();
        $rs = $stmt->fetch();

        return $rs['ID'];
    }

@ORMEntity
@ORMId
@ORMGeneratedValue(strategy="CUSTOM")
@ORMSequenceGenerator(class="AcmeBaseBundleMyIdGenerator")