subscription.rb

class Subscription < ActiveRecord::Base
  before_save :_check_internal_consistency

  def _check_internal_consistency
    if billing_period_start_date && billing_period_start_date > billing_period_end_date
      raise "Subscription internal state is inconsistent -- billing_period_start_date vs billing_period_end_date: #{inspect}"
    end

    case status
    when Status::Active
      if billing_period_start_date && _correct_next_billing_date != next_billing_date
        raise "Subscription internal state is inconsistent -- next_billing_date incorrect: #{inspect}"
      end
    when Status::PastDue
      if failure_count > MAX_RETRY_COUNT && _correct_next_billing_date != next_billing_date
        raise "Subscription internal state is inconsistent -- next_billing_date for past due subscription over max retry count: #{inspect}"
      end
    end

    ...
  end
end