Record a Transaction

Lets record a banking transaction against our bank account. Start with the TransactionService class that we haven't used to this point.

Using the method signature defined in the recordTransaction method, implement the additional logic to recording a banking transaction.

public Transaction recordTransaction(Transaction txn){
    return null;
}

Key things to implement.

• Persisting of the banking transaction
• Update the account balance as appropriate
• Define if the account has a negative balance and updated the overDraft attribute accordingly.

Relationship to Account

    @Document("transactions")
    public class Transaction {
        ...

        @DocumentReference
        @JsonBackReference
        private Account account;

        public Account getAccount() {
            return account;
        }

        public void setAccount(Account account) {
            this.account = account;
        }

Business Logic

TrasactionSevice Example

    public Transaction recordTransaction(Transaction txn){
        Account account = accountRepository.findById(txn.getAccountNumber()).get();
        BigDecimal amount = txn.getType().equals(Transaction.TransactionType.CREDIT) ? txn.getAmount() : txn.getAmount().multiply(BigDecimal.valueOf(-1L));
        txn.setAccount(account);
        Transaction mdbTxn = transactionRepository.insert(txn);
        account.setBalance(account.getBalance().add(amount));
        accountRepository.save(account);
        return mdbTxn;
    }

warning

The above code is not concurrency safe, and could result in invalid account balance entries

• Why?
• How can you fix it?

Update Operators

Example above uses the insert() method insted of save() when inserting the txn record. The insert() method maps to the insertOne or insertMany calls in MongoDB where save() is a spring method can be either insert or udpate.

It would not be appropriate for an update to an existing transaction, so insert() is the appropriate method here.

The other impact of save() is that it will attempt to update every field of the document. This can be slower performing and not idempotent, which can be critical in a banking type app.

$inc is one operator that MongoDB offers that can be used to increment a specific field in the document. It updates only the single field, and can be more performant. One way to do this is to combine the @Query and @Update annotations in the repository.

    @Query(value = "{'_id': ?0 }")
    @Update("{ '$inc' : { 'balance' : ?1 }, '$set': {'overDraft':  ?2}}")
    long updateAccountBalance(String accountId, BigDecimal amount, boolean overLimit);

This approach allows you to just update the specific fields that you want, instead of the entire document. It also doesn't require that the document be read into memory first in order to be updated. Notice that this method does not return an object, but instead a Long representing the number of successfully updated records.

note

You can also use the inc method with the MongoTemplate pattern

UpdateResult result = mongoTemplate.update(Account.class)
                .matching(where("_id").is(txn.getAccountNumber()))
                .apply(new Update().inc("accounts.$.balance", 50.00))
                .all();

Spring Data MongoDB Updating Documents

tip

The full list of MongoDB Update operators can be found MongoDB Update Operators