php_laravel_laracasts/app/Tag.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Tag extends Model
{
    const LABEL_KEY = 'name';

    protected $fillable = [self::LABEL_KEY];

    public function getRouteKeyName()
    {
        return static::LABEL_KEY;
    }

    public function posts()
    {
        return $this->belongsToMany(Post::class);
    }
}