Lookup backorder for updates with saved link

app/jobs/amend_backorder_job.rb

@@ -28,7 +28,7 @@ class AmendBackorderJob < ApplicationJob
     urls = FdcUrlBuilder.new(reference_link)
     orderer = FdcBackorderer.new(user, urls)
 
-    backorder = orderer.find_open_order
+    backorder = orderer.find_open_order(order)
 
     variants = order_cycle.variants_distributed_by(distributor)
     adjust_quantities(order_cycle, user, backorder, urls, variants)

app/models/spree/order.rb

@@ -71,6 +71,7 @@ module Spree
     has_one :exchange, ->(order) {
       outgoing.to_enterprise(order.distributor)
     }, through: :order_cycle, source: :exchanges
+    has_many :semantic_links, through: :exchange
 
     belongs_to :distributor, class_name: 'Enterprise', optional: true
     belongs_to :customer, optional: true

app/services/fdc_backorderer.rb

@@ -10,7 +10,7 @@ class FdcBackorderer
   end
 
   def find_or_build_order(ofn_order)
-    find_open_order || build_new_order(ofn_order)
+    find_open_order(ofn_order) || build_new_order(ofn_order)
   end
 
   def build_new_order(ofn_order)
@@ -19,7 +19,37 @@ class FdcBackorderer
     end
   end
 
-  def find_open_order
+  # Try the new method and fall back to old method.
+  def find_open_order(ofn_order)
+    lookup_open_order(ofn_order) || find_last_open_order
+  end
+
+  def lookup_open_order(ofn_order)
+    # There should be only one link at the moment but we may support
+    # ordering from multiple suppliers one day.
+    semantic_ids = ofn_order.semantic_links.pluck(:semantic_id)
+
+    semantic_ids.lazy
+      # Make sure we select an order from the right supplier:
+      .select { |id| id.starts_with?(urls.orders_url) }
+      # Fetch the order from the remote DFC server, lazily:
+      .map { |id| find_order(id) }
+      .compact
+      # Just in case someone completed the order without updating our database:
+      .select { |o| o.orderStatus[:path] == "Held" }
+      .first
+      # The DFC Connector doesn't recognise status values properly yet.
+      # So we are overriding the value with something that can be exported.
+      &.tap { |o| o.orderStatus = "dfc-v:Held" }
+  end
+
+  # DEPRECATED
+  #
+  # We now store links to orders we placed. So we don't need to search
+  # through all orders and pick a random open one.
+  # But for compatibility with currently open order cycles that don't have
+  # a stored link yet, we keep this method as well.
+  def find_last_open_order
     graph = import(urls.orders_url)
     open_orders = graph&.select do |o|
       o.semanticType == "dfc-b:Order" && o.orderStatus[:path] == "Held"

spec/models/spree/order_spec.rb

@@ -7,6 +7,7 @@ RSpec.describe Spree::Order do
   let(:order) { build(:order, user:) }
 
   it { is_expected.to have_one :exchange }
+  it { is_expected.to have_many :semantic_links }
 
   describe "#errors" do
     it "provides friendly error messages" do

spec/services/fdc_backorderer_spec.rb

@@ -27,7 +27,7 @@ RSpec.describe FdcBackorderer do
     # After closing the order at the end, the test can be repeated live again.
 
     # Build a new order when no open one is found:
-    order.order_cycle = build(:order_cycle)
+    order.order_cycle = create(:order_cycle, distributors: [order.distributor])
     backorder = subject.find_or_build_order(order)
     expect(backorder.semanticId).to eq urls.orders_url
     expect(backorder.lines).to eq []
@@ -50,10 +50,19 @@ RSpec.describe FdcBackorderer do
     expect(found_backorder.lines.count).to eq 1
     expect(found_backorder.lines[0].quantity.to_i).to eq 3
 
+    # Without a stored semantic link, it can't look it up directly though:
+    found_backorder = subject.lookup_open_order(order)
+    expect(found_backorder).to eq nil
+
+    # But with a semantic link, it works:
+    order.exchange.semantic_links.create!(semantic_id: placed_order.semanticId)
+    found_backorder = subject.lookup_open_order(order)
+    expect(found_backorder.semanticId).to eq placed_order.semanticId
+
     # And close the order again:
     subject.complete_order(placed_order)
     remaining_open_order = subject.find_or_build_order(order)
-    expect(remaining_open_order.semanticId).not_to eq placed_order.semanticId
+    expect(remaining_open_order.semanticId).to eq urls.orders_url
   end
 
   describe "#find_or_build_order" do