Little demonstration of Symfony2 ParamConverter on additional question on StackOverflow http://stackoverflow.com/questions/17634972/passing-objects-through-controllers-in-symfony2/17635505

MusicM_SongBundle_Controller_SongController.php

<?php

namespace MusicM\SongBundle\Controller;

use MusicM\SongBundle\Entity\Track;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

class SongController extends Controller
{

    /**
     * Load the track, based on the 'slug' field.
     * The mapping from slug to entity is automatically done by Symfony2.
     *
     * The column to use ('slug'), is defined in the parameters as cmusic_m.song.slug_field
     * in services.yml
     *
     * @see \MusicM\SongBundle\ParamConverter\EntitySlugParamConverter
     *
     * @param Track $track
     * @return \Symfony\Component\HttpFoundation\Response
     */
    public function addTrackToSongAction(Track $track)
    {
        return $this->render('MusicMSongBundle:Song:index.html.twig', array(
            'track' => $track,
        ));
    }

    /**
     * Load the track manually by slug.
     *
     * @param string $track The slug of the track
     *
     * @throws \Symfony\Component\HttpKernel\Exception\NotFoundHttpException
     * @return \Symfony\Component\HttpFoundation\Response
     */
    public function addTrackToSongManualAction($track)
    {
        $entity = $this->getDoctrine()
            ->getRepository('MusicMSongBundle:Track')
            ->findOneBySlug($track);

        if (!$entity) {
            throw new NotFoundHttpException('Track not found.');
        }

        return $this->render(
            'MusicMSongBundle:Song:index.html.twig',
            array(
                'track' => $entity,
            )
        );
    }

    public function trackListAction()
    {
        $tracks = $this->getDoctrine()
            ->getRepository('MusicMSongBundle:Track')
            ->findAll();

        return $this->render('MusicMSongBundle:Song:track_list.html.twig', array(
            'tracks' => $tracks,
        ));
    }
}

MusicM_SongBundle_DependencyInjection_Configuration.php

<?php

namespace MusicM\SongBundle\DependencyInjection;

use Symfony\Component\Config\Definition\Builder\TreeBuilder;
use Symfony\Component\Config\Definition\ConfigurationInterface;

/**
 * This is the class that validates and merges configuration from your app/config files
 *
 * To learn more see {@link http://symfony.com/doc/current/cookbook/bundles/extension.html#cookbook-bundles-extension-config-class}
 */
class Configuration implements ConfigurationInterface
{
    /**
     * {@inheritDoc}
     */
    public function getConfigTreeBuilder()
    {
        $treeBuilder = new TreeBuilder();
        $rootNode = $treeBuilder->root('music_m_song');

        // Here you should define the parameters that are allowed to
        // configure your bundle. See the documentation linked above for
        // more information on that topic.

        return $treeBuilder;
    }
}

MusicM_SongBundle_DependencyInjection_MusicMSongExtension.php

<?php

namespace MusicM\SongBundle\DependencyInjection;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\Config\FileLocator;
use Symfony\Component\HttpKernel\DependencyInjection\Extension;
use Symfony\Component\DependencyInjection\Loader;

/**
 * This is the class that loads and manages your bundle configuration
 *
 * To learn more see {@link http://symfony.com/doc/current/cookbook/bundles/extension.html}
 */
class MusicMSongExtension extends Extension
{

    public function load(array $configs, ContainerBuilder $container)
    {
        $configuration = new Configuration();
        $config = $this->processConfiguration($configuration, $configs);

        $loader = new Loader\YamlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));
        $loader->load('services.yml');
    }
}

MusicM_SongBundle_Entity_Track.php

<?php

namespace MusicM\SongBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity()
 * @ORM\Table(name="track")
 */
class Track
{

    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id()
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="slug", type="string")
     */
    private $slug;

    /**
     * @var string
     *
     * @ORM\Column(name="title", type="string")
     */
    private $title;

    /**
     * @param int $id
     */

    /**
     * @return int
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * @param string $slug
     */
    public function setSlug($slug)
    {
        $this->slug = $slug;
    }

    /**
     * @return string
     */
    public function getSlug()
    {
        return $this->slug;
    }

    /**
     * @param string $title
     */
    public function setTitle($title)
    {
        $this->title = $title;
    }

    /**
     * @return string
     */
    public function getTitle()
    {
        return $this->title;
    }
}

MusicM_SongBundle_MusicMSongBundle.php

<?php

namespace MusicM\SongBundle;

use Symfony\Component\HttpKernel\Bundle\Bundle;

class MusicMSongBundle extends Bundle
{
}

MusicM_SongBundle_ParamConverter_EntitySlugParamConverter.php

<?php

namespace MusicM\SongBundle\ParamConverter;

use Doctrine\Common\Persistence\ObjectManager;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\ParamConverter;
use Sensio\Bundle\FrameworkExtraBundle\Request\ParamConverter\ParamConverterInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

class EntitySlugParamConverter implements ParamConverterInterface
{
    protected $manager;
    protected $column;

    function __construct(ObjectManager $manager, $column = 'machine_name')
    {
        $this->manager = $manager;
        $this->column = $column;
    }

    public function apply(Request $request, ParamConverter $configuration)
    {
        $name = $configuration->getName();
        $class = $configuration->getClass();

        if (null === $request->attributes->get($name, false)) {
            $configuration->setIsOptional(true);
        }

        $entity = $this->findByIdentifierColumn($class, $request, $configuration);
        if (null === $entity && false === $configuration->isOptional()) {
            throw new NotFoundHttpException(sprintf('%s object not found.', $class));
        }

        $request->attributes->set($name, $entity);

        return $entity;
    }

    public function supports(ParamConverter $configuration)
    {
        $metadata = $this->manager->getClassMetadata($configuration->getClass());
        $hasIdentifierColumn = $metadata->hasField($this->column);

        return $hasIdentifierColumn;
    }

    protected function findByIdentifierColumn($class, Request $request, ParamConverter $configuration)
    {
        $name = $configuration->getName();
        $value = $request->attributes->get($name, false);

        if (false === $value) {
            return null;
        }

        $repository = $this->manager->getRepository($class);
        $object = $repository->findOneBy(array(
            $this->column => $value,
        ));

        return $object;
    }
}

MusicM_SongBundle_Resources_config_routing.yml

MusicMBundle_Song_addTrackToSong:
    pattern:  /tracks/add-to-song/{track}
    defaults: { _controller: MusicMSongBundle:Song:addTrackToSong }

MusicMBundle_Song_addTrackToSongManual:
    pattern:  /tracks/add-to-song/manual/{track}
    defaults: { _controller: MusicMSongBundle:Song:addTrackToSongManual }

MusicMBundle_Song_trackList:
    pattern:  /tracks/list
    defaults: { _controller: MusicMSongBundle:Song:trackList }

MusicM_SongBundle_Resources_config_services.yml

parameters:
    music_m_song.param_converter.entity_slug_param_converter.class: MusicM\SongBundle\ParamConverter\EntitySlugParamConverter
    music_m.song.slug_field: slug

services:
    music_m_song.param_converter.entity_slug_param_converter:
        class: %music_m_song.param_converter.entity_slug_param_converter.class%
        arguments: [@doctrine.orm.entity_manager, %music_m.song.slug_field%]
        tags:
            - { name: request.param_converter, priority: 1, converter: entity_slug }

MusicM_SongBundle_Resources_views_Song_index.html.twig

<h1>Track details</h1>

<dl>
    <dt>Id</dt>
    <dd>{{ track.id }}</dd>
    <dt>Title</dt>
    <dd>{{ track.title }}</dd>
</dl>

<a href="{{ path('MusicMBundle_Song_trackList') }}">&larr; Back to track list</a>

MusicM_SongBundle_Resources_views_Song_track_list.html.twig

<h1>Track list</h1>

<table>
    <tr>
        <th>Id</th>
        <th>Slug</th>
        <th>Title</th>
        <th>Details action</th>
    </tr>
    {% for track in tracks %}
    <tr>
        <td>{{ track.id }}</td>
        <td>{{ track.slug }}</td>
        <td>{{ track.title }}</td>
        <td>
            <a href="{{ path('MusicMBundle_Song_addTrackToSong', { 'track': track.slug }) }}">auto</a>
            <a href="{{ path('MusicMBundle_Song_addTrackToSongManual', { 'track': track.slug } )  }}">manual</a>
        </td>
    </tr>
    {% endfor %}
</table>