app\models\Beatmapset::unfavourite PHP Method

    public function unfavourite($user)
    {
        if (!$this->hasFavourited($user)) {
            return;
        }
        DB::transaction(function () use($user) {
            $this->favourites()->where('user_id', $user->user_id)->delete();
            $this->favourite_count = DB::raw('GREATEST(favourite_count - 1, 0)');
            $this->save();
        });
    }