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Validating Loan Payment Calculations in maybe-finance

This test suite validates the loan payment calculation functionality in the Maybe Finance application, focusing specifically on fixed-rate mortgage scenarios. The tests ensure accurate monthly payment computations based on principal, interest rate, and loan term parameters.

Test Coverage Overview

The test coverage focuses on validating the core loan calculation logic for fixed-rate mortgages. It verifies accurate monthly payment calculations using realistic loan parameters:

Principal amount: $500,000
Interest rate: 3.5%
Term: 360 months (30 years)
Expected monthly payment: $2,245

Implementation Analysis

The test implementation uses Minitest’s ActiveSupport::TestCase framework with a straightforward unit testing approach. It demonstrates proper model relationship testing between Account and Loan models, utilizing fixtures for family data references and validating financial calculations with precise assertions.

Technical Details

Test Environment Components:

Testing Framework: Minitest
Model Testing: ActiveSupport::TestCase
Fixtures: Used for family data
Data Types: Currency handling in USD
Models: Account and Loan with polymorphic association

Best Practices Demonstrated

The test exhibits several testing best practices including:

Isolated test scenarios with clear setup
Precise numeric assertions for financial calculations
Proper model relationship testing
Realistic test data values
Clear test naming conventions

maybe-finance/maybe

test/models/loan_test.rb

            
require "test_helper"

class LoanTest < ActiveSupport::TestCase
  test "calculates correct monthly payment for fixed rate loan" do
    loan_account = Account.create! \
      family: families(:dylan_family),
      name: "Mortgage Loan",
      balance: 500000,
      currency: "USD",
      accountable: Loan.create!(
        interest_rate: 3.5,
        term_months: 360,
        rate_type: "fixed"
      )

    assert_equal 2245, loan_account.loan.monthly_payment.amount
  end
end