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