// TODO EntityBuilder ???
 private Operation buildOperation(User user, String description, Double amount) {
   Operation operation = new Operation();
   operation.setUser(user);
   Set<Account> accounts = user.getAccounts();
   Account account = null;
   Iterator<Account> iterator = accounts.iterator();
   while (iterator.hasNext()) {
     account = iterator.next();
   }
   operation.setAccount(account);
   operation.setAmount(amount);
   operation.setDescription(description);
   operation.setDate(Calendar.getInstance());
   return operation;
 }