Show last post in category - symfony

Question

I just edit the topic details.
I am currently in the process of developing a Forum, which consists of 4 entity:
- Category which includes several forums
- Forum which brings together several topics
- Topic comprising several Posts
- Post which contains answers to topics
Everything works fine for now but one point at a query that I can not achieve.
I would like on the home page of the forum all the categories with sub forums to allow rapid navigation.
The problem I have is that I can not retrieve the post dernirer performed for each forum, and I would like to know how I do for solve this problem ?
So I would like to display all categories associated with all these forums and post the last post in the entity so LastPost topic of each forum.
Here's an overview of what I would do : http://hpics.li/029d17b
My repository :
public function findAllCategory()
{
$qb = $this->createQueryBuilder('c')
->leftJoin('c.forum', 'f')
->addSelect('f')
->leftJoin('f.topic', 't')
->addSelect('t')
->orderBy('c.id', 'DESC');
return $qb->getQuery()
->getResult();
}
My controler :
$categorys = $em->getRepository('AppBundle:Category')->findAllCategory();
Here is my twig file :
{% for category in categorys %}
<div>
<div class="row">
<div class="col-md-12"><h2>{{ category.title }}</h2></div>
</div>
<div class="row">
{% for forum in category.forum %}
<div class="col-md-4">
<h3>{{ forum.title }}</h3>
{{ forum.description }}
Last Post :<br/>
{% if forum.lastPost is not empty %}
{{ forum.topic.title }}
{% endif %}
</div>
{% endfor %}
</div>
</div>
{% endfor %}
My entities :
class Category
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #ORM\OneToMany(targetEntity="AppBundle\Entity\Forum", mappedBy="category")
* #ORM\JoinColumn(nullable=true)
*/
private $forum;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=255)
*/
private $title;
/**
* #Gedmo\Slug(fields={"title", "id"})
* #ORM\Column(length=128, unique=true)
*/
private $slug;
}
class Forum
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #ORM\ManyToOne(targetEntity="AppBundle\Entity\Category", inversedBy="forum")
* #ORM\JoinColumn(nullable=false)
*/
private $category;
/**
* #ORM\OneToMany(targetEntity="AppBundle\Entity\Topic", mappedBy="forum")
* #ORM\JoinColumn(nullable=true)
*/
private $topic;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=255)
*/
private $title;
/**
* #Gedmo\Slug(fields={"title", "id"})
* #ORM\Column(length=128, unique=true)
*/
private $slug;
/**
* #var string
*
* #ORM\Column(name="description", type="string", length=255)
*/
private $description;
/**
* #ORM\OneToOne(targetEntity="AppBundle\Entity\Post")
* #ORM\JoinColumn(nullable=true)
*/
private $lastPost;
}
class Topic
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #ORM\ManyToOne(targetEntity="UserBundle\Entity\User", inversedBy="topic")
* #ORM\JoinColumn(nullable=false)
*/
private $user;
/**
* #ORM\ManyToOne(targetEntity="AppBundle\Entity\Forum", inversedBy="topic")
* #ORM\JoinColumn(nullable=false)
*/
private $forum;
/**
* #ORM\OneToMany(targetEntity="AppBundle\Entity\Post", mappedBy="topic")
* #ORM\JoinColumn(nullable=true)
*/
private $post;
/**
* #var string
*
* #ORM\Column(name="title", type="string", length=255)
*/
private $title;
/**
* #Gedmo\Slug(fields={"title", "id"})
* #ORM\Column(length=128, unique=true)
*/
private $slug;
/**
* #var \DateTime
*
* #ORM\Column(name="created_at", type="datetime")
*/
private $createdAt;
/**
* #ORM\OneToOne(targetEntity="AppBundle\Entity\Post")
* #ORM\JoinColumn(nullable=true)
*/
private $lastPost;
}
class Post
{
/**
* #var integer
*
* #ORM\Column(name="id", type="integer")
* #ORM\Id
* #ORM\GeneratedValue(strategy="AUTO")
*/
private $id;
/**
* #ORM\ManyToOne(targetEntity="UserBundle\Entity\User", inversedBy="post")
* #ORM\JoinColumn(nullable=false)
*/
private $user;
/**
* #ORM\ManyToOne(targetEntity="AppBundle\Entity\Topic", inversedBy="post")
* #ORM\JoinColumn(nullable=false)
*/
private $topic;
/**
* #var string
*
* #ORM\Column(name="message", type="text")
*/
private $message;
/**
* #var \DateTime
*
* #ORM\Column(name="created_at", type="datetime")
*/
private $createdAt;
}
Thanks !