PostController.php

<?php

namespace App\Http\Controllers\Api\v1;

use Illuminate\Http\Request;
use App\Http\Controllers\Controller;

class PostController extends Controller
{
    public function index() 
    {
      $postRepository = new PostRepository(new Post);
      
      $posts = $postRepository->listAllPosts('desc');
      $posts->categories = $postRepository->listCategories();
      
      return response()->json([
        'data' => $posts,
        'status' => 200
      ], 200)
    }
}

//Post Repository
namespace App;

use App\Post;
use Illuminate\Support\Collection;

class PostRepository {
  
  protected $model;
  
  public function __construct(Post $post) 
  {
    $this->model = $post;
  }
  
  /**
  *
  * Return all the posts
  * @param string $sort
  * @return Collection
  */
  public function listAllPosts($sort = 'desc') : Collection
  {
    return $this->model->orderBy('id', $sort)->get();
  }
  
  /**
  *
  * Return all related categories of the post
  * @return Collection
  */
  public function listCategories() : Collection
  {
    return $this->model->categories()->get();
  }
}

// Post Model
namespace App;

use App\Category;
use Illuminate\Database\Eloquent\Model;

class Post extends Model {
  ....
  public function categories() 
  {
    return $this->belongsToMany(Category::class);
  }
}