Many to Many Query

Post.php

<?php

namespace ContentBundle\Entity;

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

/**
 * @ORM\Entity
 */
class Post
{
    /**
     * @ORM\Column(type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @ORM\Column(type="string")
     */
    protected $title;

    /**
     * @ORM\ManyToMany(targetEntity="Tag",inversedBy="tags",cascade={"persist", "remove"})
     * @ORM\JoinTable(name="post_tags")
     */
    protected $tags;

    public function __construct() {
        $this->tags = new ArrayCollection();
    }

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

    /**
     * @param int $id
     */
    public function setId($id)
    {
        $this->id = $id;
    }

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

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

    public function addTag(Tag $tag)
    {
        $tag->addPost($this);
        $this->tags[] = $tag;
    }

    public function getTags()
    {
        return $this->tags;
    }

}

query.php

<?php

namespace AppBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;

class DefaultController extends Controller
{
    public function indexAction(Request $request)
    {
        $qb = $this->getDoctrine()->getManager()->createQueryBuilder()
            ->select('t AS tag')
            ->addSelect('COUNT(p) AS num_posts')
            ->from('ContentBundle:Tag', 't')
            ->leftJoin('ContentBundle:Post', 'p', 'WITH', 't MEMBER OF p.tags')
            ->groupBy('t.id');

        dump($qb->getQuery()->getResult());
    }
}

return.txt

array:2 [▼
  0 => array:2 [▼
    "tag" => Tag {#400 ▶}
    "num_posts" => "2"
  ]
  1 => array:2 [▼
    "tag" => Tag {#406 ▶}
    "num_posts" => "1"
  ]
]

Tag.php

<?php

namespace ContentBundle\Entity;

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

/**
 * Tag
 *
 * @ORM\Table()
 * @ORM\Entity
 */
class Tag
{
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="name", type="string", length=255)
     */
    private $name;

    /**
     * @ORM\ManyToMany(targetEntity="Post", mappedBy="tags")
     */
    private $posts;

    public function __construct($name = null)
    {
        $this->posts = new ArrayCollection();
        $this->setName($name);
    }

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

    /**
     * Set name
     *
     * @param string $name
     * @return Tag
     */
    public function setName($name)
    {
        $this->name = $name;

        return $this;
    }

    /**
     * Get name
     *
     * @return string 
     */
    public function getName()
    {
        return $this->name;
    }

    public function addPost(Post $post)
    {
        $this->posts[] = $post;
    }
}