PostController.php

<?php

namespace App\Http\Controllers\Ajax;

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

class PostController extends Controller
{
    public function index()
    {
        return Datatables::of(Post::query()->with('comments', 'user'))
        ->addColumn('user_name', function($row){
            return $row->user->name;
        })
        ->addColumn('comments_num', function($row){
            return $row->comments->count();
        })
        ->make(true);
    }
}