php_laravel_laracasts/app/Post.php

<?php

namespace App;

use Illuminate\Http\Request;

class Post extends Model {

  /**
   * These fields are OK for mass assignement.
   *
   * @var array
   */
  protected $fillable = ['body', 'title'];

    /**
     * @param string $body
     */
    public function addComment(string $body) {
//      Comment::create([
//          'body' => $body,
//          'post_id' => $this->id,
//      ]);

        $this->comments()->create(compact('body'));
  }

  public function comments()
  {
      return $this->hasMany(Comment::class);
  }
}