一个天真的解决方案看起来像这样.

services.yML

services:
    invoice.listener:
        class: MyCompany\CompanyBundle\EventListener\InvoiceListener
        tags :
            - { name: doctrine.event_subscriber,connection: default }

InvoiceListener.PHP

<?PHP

namespace MyCompany\CompanyBundle\EventListener;

use Symfony\component\EventDispatcher\EventDispatcherInterface;
use Doctrine\Common\EventSubscriber;
use Doctrine\ORM\Event\OnFlushEventargs;
use Doctrine\ORM\Event\PostFlushEventArgs;

use MyCompany\CompanyBundle\EntITy\Invoice;

class InvoiceListener implements EventSubscriber {

    PRotected $invoices;

    public function getSubscribedEvents() {
        return [
            'onFlush','postFlush'
        ];
    }

    public function onFlush(OnFlushEventArgs $event) {
        $this->invoices = [];
        /* @VAR $em \Doctrine\ORM\EntityManager */
        $em = $event->getEntityManager();
        /* @var $uow \Doctrine\ORM\UnitOfWork */
        $uow = $em->getUnitOfWork();

        foreach ($uow->getScheduledEntityInsertions() as $entity) {
            if ($entity instanceof Invoice) {
                $this->invoices[] = $entity;
            }
        }
    }

    public function postFlush(PostFlushEventArgs $event) {
        if (!empty($this->invoices)) {

            /* @var $em \Doctrine\ORM\EntityManager */
            $em = $event->getEntityManager();

            foreach ($this->invoices as $invoice) {
                // Get all invoices already in the database for the year in question
                $invoicesToDate = $em
                    ->getRepository('MyCompanyCompanyBundle:Invoice')
                    ->findBy(array(
                        'year' => $invoice->getYear()
                        // You Could include e.g. clientID here if you wanted
                        // to generate a different sequence PEr client
                    );
                // Add your sequence number
                $invoice->setSequenceNum(count($invoicesToDate) + 1);

                /* @var $invoice \MyCompany\CompanyBundle\Entity\Invoice */
                $em->persist($invoice);
            }

            $em->flush();
        }
    }
}