BlogController.php

<?php

class BlogController extends Controller
{
    /**
     * @Route("/comment/{postSlug}/new", name = "comment_new")
     * @Security("is_granted('IS_AUTHENTICATED_FULLY')")
     *
     * @Method("POST")
     * @ParamConverter("post", options={"mapping": {"postSlug": "slug"}})
     */
    public function commentAction(Request $request, Post $post)
    {
        $form = $this->createCommentForm();
        $form->handleRequest($request);
        
        if ($form->isSubmitted() && $form->isValid()) {
 
            $this->getUser()->comment($post, $form->get('content')->getData());
            $this->getDoctrine()->getManager()->flush();
            
            return $this->redirectToRoute('blog_post', array('slug' => $post->getSlug()));
        }
        
        return $this->render('blog/comment_form_error.html.twig', array(
            'post' => $post,
            'form' => $form->createView(),
        ));
    }
}

Comment.php

<?php

class Comment
{
    private $author;
    
    private $content;
    
    private $createdAt;
    
    private $lastUpdatedAt;
    
    public function __construct(User $author, Content $content, \DateTime $createdAt)
    {
        $this->author = $author;
        $this->content = $content;
        $this->createdAt = $createdAt;
    }
    
    public function wasCreatedBy(User $user)
    {
        return $this->user->isEqualTo($user);
    }
    
    public function updateContent(Content $newContent)
    {
        $this->content = $newContent; 
        $this->lastUpdatedAt = new \DateTime();
    }
}

Content.php

<?php

class Content
{
    private $value; 
    
    public function __construct($value)
    {
        if (empty($value)) {
            throw new \InvalidArgumentException("Post comment content can't be empty.");
        }
        
        $this->value = $value;
    }
    
    public function __toString()
    {
        return $this->value;
    }
}

Post.php

<?php

class Post
{
    public function addComment(Content $content, User $author)
    {
        $this->comments->add(new Comment($author, $content, new DateTime()));
    }
}

User.php

<?php

class User 
{
    public function comment(Post $post, $content)
    {
        if (!$post->canBeCommented()) {
            throw new \RuntimeException("Post can't be commented.");
        }
        
        $post->addComment(new Content($content), $this);
    }
    
    public function updateComment(Comment $comment, $content)
    {
        if ($comment->wasCreatedByUser($this)) {
            throw new \RuntimeException("You can not update not your comments.");
        }   
        
        $comment->updateContent(new Content($content));
    }
}