Posted onDec 30, 2024 • Originally published atsulmanweb.com

Rails Testing for Financial Operations

Testing financial applications requires exceptional attention to detail and robust test coverage. In this article, we'll explore advanced testing patterns for financial operations using a real-world Rails application. We'll cover transaction testing, balance validations, and audit logging verification.

The Foundation: Service Objects and RSpec

Our financial application uses a service-object pattern to encapsulate business logic. Here's how we structure our tests:

RSpec.describeTransactions::CreateServicedosubject(:service){described_class.new(params)}let(:user){create(:user)}let(:account){create(:account,balance:100,user:user)}let(:params)do{user_id:user.id,account_id:account.id,amount:50,transaction_type:'expense'}endend

Testing Financial Transactions

When testing financial transactions, we need to verify several aspects:

Balance updates
Transaction records
Audit logs
Error handling

Here's a comprehensive test example:

describe'#call'docontext'when creating an expense transaction'doit'updates the account balance correctly'doresult=service.callexpect(result).tobe_successexpect(account.reload.balance).toeq(50)# 100 - 50endit'creates an audit log'doexpect{service.call}.tochange(AuditLog,:count).by(1)endendcontext'when creating an income transaction'dolet(:params)do{user_id:user.id,account_id:account.id,amount:50,transaction_type:'income'}endit'increases the account balance'doresult=service.callexpect(result).tobe_successexpect(account.reload.balance).toeq(150)# 100 + 50endendend

Testing Money Transfers

Transfer operations are particularly critical as they involve multiple accounts. Here's how we test them:

RSpec.describeTransactions::TransferServicedolet(:user){create(:user)}let(:account_from){create(:account,user:user,balance:1000)}let(:account_to){create(:account,user:user,balance:0)}let(:params)do{user_id:user.id,account_from_id:account_from.id,account_to_id:account_to.id,amount:100}enddescribe'#call'doit'transfers money between accounts'doservice=described_class.new(params)result=service.callexpect(result).tobe_successexpect(account_from.reload.balance).toeq(900)expect(account_to.reload.balance).toeq(100)endit'creates two transactions'doexpect{service.call}.tochange(Transaction,:count).by(2)endendend

Shared Examples for Common Behaviors

To maintain DRY tests, we use shared examples for common behaviors:

RSpec.shared_examples'an audit log'do|service_call,should_create|ifshould_createit'creates an audit log'doexpect{instance_exec(&service_call)}.tochange(AuditLog,:count).by(1)endelseit'does not create an audit log'doexpect{instance_exec(&service_call)}.not_tochange(AuditLog,:count)endendend# Usage in specsdescribe'successful transaction'doinclude_examples'an audit log',->{service.call},trueend

Testing Edge Cases

Financial applications must handle edge cases gracefully:

describe'edge cases'docontext'with insufficient funds'dolet(:account){create(:account,balance:10)}let(:params)do{user_id:user.id,account_id:account.id,amount:100,transaction_type:'expense'}endit'fails the transaction'doresult=service.callexpect(result).not_tobe_successexpect(account.reload.balance).toeq(10)endendcontext'with invalid amounts'dolet(:params)do{user_id:user.id,account_id:account.id,amount:-50,transaction_type:'expense'}endit'rejects negative amounts'doresult=service.callexpect(result).not_tobe_successendendend

Testing Currency Conversions

When dealing with multiple currencies, we need to test conversion accuracy:

RSpec.describeCurrencydolet(:usd){create(:currency,code:'USD',amount:1.0)}let(:eur){create(:currency,code:'EUR',amount:0.85)}describe'currency conversion'doit'converts amounts correctly'doaccount=create(:account,currency:eur,balance:100)# Verify USD equivalentexpect(account.balance_in_usd).toeq(117.65)# 100 / 0.85endendend

Testing GraphQL Mutations

Our financial operations are exposed via GraphQL. Here's how we test them:

RSpec.describeMutations::TransactionCreatedolet(:user){create(:user)}let(:account){create(:account,user:user)}let(:query)do<<~GQL      mutation($input: TransactionCreateInput!) {        transactionCreate(input: $input) {          success          transaction { id amount }        }      }    GQLendit'creates a transaction through GraphQL'dovariables={input:{accountId:account.id,amount:100,transactionType:'EXPENSE'}}post'/graphql',params:{query:query,variables:variables.to_json},headers:auth_headers(user)expect(response.parsed_body['data']['transactionCreate']).toinclude('success'=>true)endend

Best Practices and Recommendations

Always test in a transaction block to ensure database cleanliness
Use factory_bot for test data setup
Test both happy and unhappy paths
Verify audit logs for sensitive operations
Use decimal for money calculations to avoid floating-point errors
Test currency conversions with known exchange rates
Verify transaction atomicity in transfers
Test authorization and authentication

Conclusion

Testing financial operations requires a comprehensive approach that covers not just the happy path but also edge cases, error conditions, and audit requirements. By following these patterns and practices, you can build reliable financial applications with confidence in their correctness.

Remember that financial data is critical, and tests are your first line of defense against bugs and inconsistencies. Take the time to write thorough tests, and your future self (and your users) will thank you.

This testing approach has been battle-tested in production environments and provides a solid foundation for building robust financial applications. The key is to think about all possible scenarios and edge cases while maintaining clean, readable, and maintainable tests.

Happy Coding!

Originally published athttps://sulmanweb.com.