$user = User::findOrFail($uuid);
$book = Books::where('isbn', '=', $isbn)->take(1)->get();
if (!$user->userBookLinks()->contains($book->id) && $user->borrowLinks->count() < 5) {
$user->userBookLinks()->attach($book->id, ['date_due' => new DateTime('+2 Weeks')];
}
$user->save();