Amend backorder after cancellations

The new job class blends code from the BackorderJob and the
CompleteBackorderJob for the specific case of adjusting quantities after
an order has been cancelled.

I would like to write a more general class which can be used for any
order amendmends but this was the quickest solution to cater for
currently running pilots.

app/jobs/amend_backorder_job.rb

@@ -0,0 +1,98 @@
+# frozen_string_literal: true
+
+# When orders are cancelled, we need to amend
+# an existing backorder as well.
+# We're not dealing with line item changes just yet.
+class AmendBackorderJob < ApplicationJob
+  sidekiq_options retry: 0
+
+  def perform(order)
+    OrderLocker.lock_order_and_variants(order) do
+      amend_backorder(order)
+    end
+  end
+
+  # The following is a mix of the BackorderJob and the CompleteBackorderJob.
+  # TODO: Move the common code into a re-usable service class.
+  def amend_backorder(order)
+    order_cycle = order.order_cycle
+    distributor = order.distributor
+    user = distributor.owner
+    items = backorderable_items(order)
+
+    return if items.empty?
+
+    # We are assuming that all variants are linked to the same wholesale
+    # shop and its catalog:
+    reference_link = items[0].variant.semantic_links[0].semantic_id
+    urls = FdcUrlBuilder.new(reference_link)
+    orderer = FdcBackorderer.new(user, urls)
+
+    backorder = orderer.find_open_order
+
+    variants = order_cycle.variants_distributed_by(distributor)
+    adjust_quantities(order_cycle, user, backorder, urls, variants)
+
+    FdcBackorderer.new(user, urls).send_order(backorder)
+  end
+
+  # Check if we have enough stock to reduce the backorder.
+  #
+  # Our local stock can increase when users cancel their orders.
+  # But stock levels could also have been adjusted manually. So we review all
+  # quantities before finalising the order.
+  def adjust_quantities(order_cycle, user, order, urls, variants)
+    broker = FdcOfferBroker.new(user, urls)
+
+    order.lines.each do |line|
+      line.quantity = line.quantity.to_i
+      wholesale_product_id = line.offer.offeredItem.semanticId
+      transformation = broker.wholesale_to_retail(wholesale_product_id)
+      linked_variant = variants.linked_to(transformation.retail_product_id)
+
+      # Assumption: If a transformation is present then we only sell the retail
+      # variant. If that can't be found, it was deleted and we'll ignore that
+      # for now.
+      next if linked_variant.nil?
+
+      # Find all line items for this order cycle
+      # Update quantity accordingly
+      if linked_variant.on_demand
+        release_superfluous_stock(line, linked_variant, transformation)
+      else
+        aggregate_final_quantities(order_cycle, line, linked_variant, transformation)
+      end
+    end
+
+    # Clean up empty lines:
+    order.lines.reject! { |line| line.quantity.zero? }
+  end
+
+  # We look at all linked variants.
+  def backorderable_items(order)
+    order.line_items.select do |item|
+      # TODO: scope variants to hub.
+      # We are only supporting producer stock at the moment.
+      item.variant.semantic_links.present?
+    end
+  end
+
+  def release_superfluous_stock(line, linked_variant, transformation)
+    # Note that a division of integers dismisses the remainder, like `floor`:
+    wholesale_items_contained_in_stock = linked_variant.on_hand / transformation.factor
+
+    # But maybe we didn't actually order that much:
+    deductable_quantity = [line.quantity, wholesale_items_contained_in_stock].min
+    line.quantity -= deductable_quantity
+
+    retail_stock_changes = deductable_quantity * transformation.factor
+    linked_variant.on_hand -= retail_stock_changes
+  end
+
+  def aggregate_final_quantities(order_cycle, line, variant, transformation)
+    orders = order_cycle.orders.invoiceable
+    quantity = Spree::LineItem.where(order: orders, variant:).sum(:quantity)
+    wholesale_quantity = (quantity.to_f / transformation.factor).ceil
+    line.quantity = wholesale_quantity
+  end
+end

app/models/spree/order/checkout.rb

@@ -142,6 +142,8 @@ def after_cancel
 
             OrderMailer.cancel_email(id).deliver_later if send_cancellation_email
             update(payment_state: updater.update_payment_state)
+
+            AmendBackorderJob.perform_later(self)
           end
 
           def after_resume