spree/spree-product-assembly

module Spree::InventoryUnitDecorator
  def percentage_of_line_item
    product = line_item.product
    if product.assembly?
      total_value = line_item.quantity_by_variant.map { |part, quantity| part.price * quantity }.sum
      variant.price / total_value
    else
      1 / BigDecimal(line_item.quantity)
    end
  end
end

Spree::InventoryUnit.prepend Spree::InventoryUnitDecorator