Validate Promotions

checkout_controller_decorator.rb

Spree::CheckoutController.class_eval do
  require_dependency 'spree/order_decorator'

  rescue_from Spree::Order::CouldNotValidateMailingListPromoCode do |exception|
    flash[:error] = t(exception.message)
    redirect_to checkout_state_path(@order.state)
  end

end

order_decorator.rb

Spree::Order.class_eval do

  def validate_coupons
    multiple_promos = false

    adjustments.reorder('amount ASC').promo.each_with_index do |adjustment, index|
      unless index == 0
        adjustment.destroy 
        save!
        multiple_promos = true
      end
    end
    raise CouldNotValidateMailingListPromoCode, 'promotion_rule_types.on_mailing_list.used_promo' if multiple_promos
  end

  def process_payments!
    validate_coupons #this method call should be placed elsewhere
    ret = payments.each(&:process!)
  end

  class Spree::Order::CouldNotValidateMailingListPromoCode < StandardError
  end

end