What is a good approach to save relations in repository pattern?

I'm confused about using repository pattern when it comes to saving data. I've written discussion board so when user creates a new thread I need to save many objects (relations). Basically I need to:

1. Create new record in topics table
2. Create new record in posts table
3. Assign post's attachments
4. Add user ID to subscribers table (so he/she can recieve notifications about new replies)

It might look like this:

$topic = $topicRepository->create($request->all()); // $topic is instance of Eloquent model
$post = $postRepository->create($request->all() + ['topic_id' => $topic->id]); 

// ... save attachments etc

$url = route('topic', [$topic->id, $post->id]) . '#post' . $post->id; // build URL to new post

// I have Topic and Post model: creating notification ...

BUT I have feeling I'm doing it wrong. Shouldn't I create new method in my repository that can create new thread (add new records to topics, posts tables) and keep my controller clean?

Maybe like this?

// TopicRepository.php:

public function createWithPost($request)
{
    $topic = Topic::create($request);
    $post = $topic->posts()->create($request);

    // ...

    return $post;
}

// Topic.php (model):

class Topic extends Eloquent
{
    public function posts()
    {
        return $this->hasMany('Post');
    }
}

// Post.php (model);

class Post extends Eloquent
{
    public function topic()
    {
        return $this->belongsTo('Topic');
    }
}


// controller:

$post = $topicRepository->createWithPost($request->all()); // $post is instance of Eloquen model
$url = route('topic', [$post->topic()->id, $post->id]) . '#post' . $post->id; // build URL to new post

So questions are:

What is a good approach to save relations in repository pattern?

Should repository pattern deal with saving all relations?

Route model binding and repository pattern

Route model binding is a great feature in Laravel. Doesn't it break repository pattern rules? I mean: should we write:

public function index($topicId, $postId)
{
    $topic = $topicRepository->findOrFail($topicId);
    $post = $postRepository->findOrFail($postId);
}

instead of:

// Topic is a instance of Eloquent model
public function index(Topic $topic, $post Post)
{
    //
}

Thank you in advance for all opinions and tips.