class Invitation extends Model
{
    protected $primaryKey = 'invite_code';

    protected $fillable = ['invite_code', 'creator_id', 'expires_at'];
    protected $dates = ['created_at', 'updated_at', 'expires_at'];

    public function getRouteKey()
    {
        return $this->attributes['invite_code'];
    }

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

    public function creator()
    {
        return $this->belongsTo(User::class, 'creator_id');
    }
}

Route::get('/test/{id}', function($id) {
    $i = AppInvitation::find($id);
    dump( $i->invite_code );
});

Schema::create('invitations', function (Blueprint $table) {
    $table->string('invite_code')->primary();
    $table->integer('creator_id');
    $table->integer('user_id');
    $table->timestamps();
    $table->dateTime('expires_at');
});