Maybe/app/models/loan.rb

class Loan < ApplicationRecord
  include Accountable

  def monthly_payment
    return nil if term_months.nil? || interest_rate.nil? || rate_type.nil? || rate_type != "fixed"
    return Money.new(0, account.currency) if account.original_balance.amount.zero? || term_months.zero?

    annual_rate = interest_rate / 100.0
    monthly_rate = annual_rate / 12.0

    if monthly_rate.zero?
      payment = account.original_balance.amount / term_months
    else
      payment = (account.original_balance.amount * monthly_rate * (1 + monthly_rate)**term_months) / ((1 + monthly_rate)**term_months - 1)
    end

    Money.new(payment.round, account.currency)
  end
end