exists()) {
    throw new InvalidAccountException();
  }

  $toQuery = Account::whereId($toAccountId);
  if (! $toQuery->exists()) {
    throw new InvalidAccountException();
  }

  do {
    $fromAccount = $fromQuery->first();
    if ($fromAccount->balance < $amount) {
      throw new InsufficientBalanceException();
    }
    $updated = Account::whereId($fromAccountId)
      ->where('updated_at', '=', $fromAccount->updated_at)
      ->update(['balance' => $fromAccount->balance - $amount]);
  } while (! $updated);

  do {
    $toAccount = $toQuery->first();    
    $updated = Account::whereId($toAccountId)
      ->where('updated_at', '=', $toAccount->updated_at)
      ->update(['balance' => $toAccount->balance + $amount]);
  } while (! $updated);

  $transaction = new Transaction();
  $transaction->from_account_id = $fromAccountId;
  $transaction->to_account_id   = $toAccountId;
  $transaction->amount          = $amount;
  $transaction->save();
}