Symfony2, Dynamic DB Connection/Early override of Doctrine Service

Combined, these two postings helped me solve my own very similar problem. Here is my solution, maybe it is useful for someone else:

<?php

namespace Calitarus\CollaborationBundle\EventListener;

use Symfony\Component\HttpFoundation\Request;
use Doctrine\DBAL\Connection;
use Exception;
use Monolog\Logger;



class DatabaseSwitcherEventListener {

    private $request;
    private $connection;
    private $logger;

    public function __construct(Request $request, Connection $connection, Logger $logger) {
        $this->request = $request;
        $this->connection = $connection;
        $this->logger = $logger;
    }


    public function onKernelRequest() {
        if ($this->request->attributes->has('_site')) {
            $site = $this->request->attributes->get('_site');

            $connection = $this->connection;
            $params     = $this->connection->getParams();

            $db_name="br_".$this->request->attributes->get('_site');
            // TODO: validate that this site exists
            if ($db_name != $params['dbname']) {
                $this->logger->debug('switching connection from '.$params['dbname'].' to '.$db_name);
                $params['dbname'] = $db_name;
                if ($connection->isConnected()) {
                    $connection->close();
                }
                $connection->__construct(
                    $params, $connection->getDriver(), $connection->getConfiguration(),
                    $connection->getEventManager()
                );

                try {
                    $connection->connect();
                } catch (Exception $e) {
                    // log and handle exception
                }
            }
        }
    }
}

To get this to work, I set up services.yml as follows:

services:
    cc.database_switcher:
        class:      Calitarus\CollaborationBundle\EventListener\DatabaseSwitcherEventListener
        arguments:  [@request, @doctrine.dbal.default_connection, @logger]
        scope:      request
        tags:
            - { name: kernel.event_listener, event: kernel.request, method: onKernelRequest }

and I have this routing configuration to get the _site parameter, which in my case is part of the URL, but you can probably get it in other ways depending on your setup:

resource: "@CCollabBundle/Controller"
type:     annotation
prefix:   /{_site}
defaults:
 _site: default