Calculator

gistfile1.rb

# models/active_debt.rb
class ActiveDebt < ActiveRecord::Base
  delegate :fine, to: :calculator, prefix: :updated

  def calculator
    LatePaymentPenalty.new self
  end
end

# Use:
# active_debt.updated_fine
# active_debt.updated_fine Date.tomorrow
 
# business/late_payment_penalty.rb
class LatePaymentPenalty
  attr_reader :debt

  delegate :value, :due_date, to: :debt

  def initialize(debt)
    @debt = debt
  end

  def fine(base_date = Date.current)
    fine_calculator.new value, from: due_date, to: base_date
  end
end