Monkey Patch to make Spree skips inventory check for only a kind of products

packer_decorator.rb

Spree::Stock::Packer.class_eval do
    def default_package
        package = Spree::Stock::Package.new(stock_location, order)
        order.line_items.each do |line_item|
            # on_demand? will say if this kind of product has inventory
            if Spree::Config.track_inventory_levels and not line_item.variant.product.on_demand?
                next unless stock_location.stock_item(line_item.variant)

                on_hand, backordered = stock_location.fill_status(line_item.variant, line_item.quantity)
                package.add line_item.variant, on_hand, :on_hand if on_hand > 0
                package.add line_item.variant, backordered, :backordered if backordered > 0
            else
                package.add line_item.variant, line_item.quantity, :on_hand
            end
        end
        package
    end
end

quantifier_decorator.rb

Spree::Stock::Quantifier.class_eval do
    def total_on_hand
        # this is needed because @variant stores only the variant id
        variant = Spree::Variant.find(@variant)

        # on_demand? will say if this kind of product has inventory
        if Spree::Config.track_inventory_levels and not variant.product.on_demand?
            stock_items.to_a.sum(&:count_on_hand)
        else
            Float::INFINITY
        end
    end
end