order_decorator.rb

  # Updates the +payment_state+ attribute according to the following logic:
  #
  # paid          when +payment_total+ is equal to +total+
  # balance_due   when +payment_total+ is less than +total+
  # credit_owed   when +payment_total+ is greater than +total+
  # failed        when most recent payment is in the failed state
  #
  # The +payment_state+ value helps with reporting, etc. since it provides a quick and easy way to locate Orders needing attention.
  def update_payment_state
    if round_money(payment_total) < round_money(total)
      self.payment_state = "balance_due"
      self.payment_state = "failed" if payments.present? and payments.last.state == "failed"
    elsif round_money(payment_total) > round_money(total)
      self.payment_state = "credit_owed"
    else
      self.payment_state = "paid"
    end

    if old_payment_state = self.changed_attributes["payment_state"]
      self.state_events.create({
        :previous_state => old_payment_state,
        :next_state     => self.payment_state,
        :name           => "payment" ,
        :user_id        =>  (User.respond_to?(:current) && User.current && User.current.id) || self.user_id
      })
    end
  end

  # round the money to 2 decimal places
  def round_money(n)
    (n*100).round / 100.0
  end