gistfile1.txt

createBill = (amount) {
  return Bill({
    amount: amount,
  });
}

createCheck = (account, bill) {
  return Check({
    accountId: account.id,
    billId: bill.id
  });
}

withdrawal = (account, bill) {
  return Account({
    ...account,
    balance: account.balance - bill.amount
  });
}

PaymentService {
  Check pay(account, amount) {
    bill = createBill(amount);
    check = createCheck(account, bill);

    account = withdrawal(account, bill);

    account.save();
    bill.save();
    check.save();

    return check;
  }
}