Using EntityRepository::findBy() with Many-To-Many relations will lead to a E_NOTICE in Doctrine

Question

For a Symfony2 project I had to create a relationship between a blog post and so called platforms. A platform defines a specific filter based on the domain you use to view the site. For example: If you join the site by url first-example.com, the site will only provide blog posts, which are connected to this specific platform.

To do so, I created two entities Post and Platform. Afterwards I mapped them together with a Many-To-Many relationship. I'm trying to retrieve data via this Many-To-Many relationship from the builtin function findBy() in Doctrines' EntityRepository.

// every one of these methods will throw the same error
$posts = $postRepo->findBy(array('platforms' => array($platform)));
$posts = $postRepo->findByPlatforms($platform);
$posts = $postRepo->findByPlatforms(array($platform));

Where $postRepo is the correct Repository for the Post entity and $platform an existing Platform object.
Either way: I end up getting the following error:

ErrorException: Notice: Undefined index: joinColumns in [...]/vendor/doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php line 1495

[...]/vendor/doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:1495
[...]/vendor/doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:1452
[...]/vendor/doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:1525
[...]/vendor/doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:1018
[...]/vendor/doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php:842
[...]/vendor/doctrine/orm/lib/Doctrine/ORM/EntityRepository.php:157
[...]/src/Foobar/BlogBundle/Tests/ORM/PostTest.php:102

Is it even possible to retrieve related entites in a Many-To-Many relationship this way, or am i forced to write these functions by myself? The weird thing is: Doctrine will not throw any error like: "It's not possible.", but an internal E_NOTICE. Thats why I tent to think it should be possible, but I'm missing some points here.

Stripped down to the interesting parts, the two Entities look like this.

<?php

namespace Foobar\CommunityBundle\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;

// [...] other namespace stuff

/**
 * @ORM\Entity(repositoryClass="Foobar\CommunityBundle\Entity\Repository\PlatformRepository")
 * @ORM\Table(name="platforms")
 */
class Platform
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    // [...] other field stuff
}

<?php

namespace Foobar\BlogBundle\Entity;

use Doctrine\Common\Collections\ArrayCollection;
use Doctrine\ORM\Mapping as ORM;

// [...] other namespace stuff

/**
 * @ORM\Entity(repositoryClass="Foobar\BlogBundle\Entity\Repository\PostRepository")
 * @ORM\Table(name="posts")
 */
class Post implements Likeable, Commentable, Taggable, PlatformAware
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @ORM\ManyToMany(targetEntity="Foobar\CommunityBundle\Entity\Platform", cascade={"persist"})
     * @ORM\JoinTable(name="map_post_platform",
     *      joinColumns={@ORM\JoinColumn(name="post_id", referencedColumnName="id")},
     *      inverseJoinColumns={@ORM\JoinColumn(name="platform_id", referencedColumnName="id")}
     *      )
     */
    protected $platforms;

    // [...] other fields

    /**
     * Constructor
     */
    public function __construct()
    {
        // [...]
        $this->platforms  = new ArrayCollection();
    }
}

And of course the composer.json file (as well stripped down to the relevant lines)

{
    [...]
    "require": {
        "php": ">=5.3.3",
        "symfony/symfony": "2.1.*",
        "doctrine/orm": ">=2.2.3,<2.4-dev",
        "doctrine/doctrine-bundle": "1.0.*",
        "doctrine/doctrine-fixtures-bundle": "dev-master",
        [...]

    },
    [...]
}

Answer 1

Another way, maybe a bit OO/cleaner without using IDs:

public function getPosts(Platform $platform)
{
    $qb = $this->createQueryBuilder("p")
        ->where(':platform MEMBER OF p.platforms')
        ->setParameters(array('platform' => $platform))
    ;
    return $qb->getQuery()->getResult();
}

A better method name would be findPostsByPlatform