Dont allow more than one promotional item in a Spree Order

migration.rb

add_column :spree_products, :promotional, :boolean

order_populator_decorator.rb

Spree::OrderPopulator.class_eval do
  def attempt_cart_add(variant_id, quantity)
    quantity = quantity.to_i
    # 2,147,483,647 is crazy.
    # See issue #2695.
    if quantity > 2_147_483_647
      errors.add(:base, Spree.t(:please_enter_reasonable_quantity, :scope => :order_populator))
      return false
    end

    variant = Spree::Variant.find(variant_id)
    if quantity > 0

      ### CUSTOM CODE - BEGIN
      if variant.promotional? and @order.line_items.any? {|line_item| line_item.variant.promotional?}
        errors.add(:base, :promotional_error_message)
        return false
      end
      ### CUSTOM CODE - END

      line_item = @order.contents.add(variant, quantity, currency)
      unless line_item.valid?
        errors.add(:base, line_item.errors.messages.values.join(" "))
        return false
      end
    end
  end
end

variant_decorator.rb

Spree::Variant.class_eval do
  def promotional?
    variant.product.promotional?
  end
end