php_laravel_laracasts/app/Post.php

<?php

namespace App;

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);
    }

    public function user()
    {
        return $this->belongsTo(User::class);
    }
}