spree-contrib/spree-product-assembly

module Spree
  module Stock
    module InventoryUnitBuilderDecorator
      def units
        @order.line_items.flat_map do |line_item|
          line_item.quantity_by_variant.flat_map do |variant, quantity|
            if Gem.loaded_specs['spree_core'].version >= Gem::Version.create('3.3.0')
              build_inventory_unit(variant, line_item, quantity)
            else
              quantity.times.map { build_inventory_unit(variant, line_item) }
            end
          end
        end
      end

      def build_inventory_unit(variant, line_item, quantity=nil)
        @order.inventory_units.includes(
          variant: {
            product: {
              shipping_category: {
                shipping_methods: [:calculator, { zones: :zone_members }]
              }
            }
          }
        ).build(
          pending: true,
          variant: variant,
          line_item: line_item,
          order: @order
        ).tap do |iu|
          iu.quantity = quantity if quantity
        end
      end
    end
  end
end

Spree::Stock::InventoryUnitBuilder.prepend Spree::Stock::InventoryUnitBuilderDecorator