Fix issue with splitting backordered units

core/app/models/spree/fulfilment_changer.rb

@@ -89,12 +89,18 @@ def run_tracking_inventory
       available_quantity = get_available_quantity
       new_on_hand_quantity = [available_quantity, quantity].min
       backordered_quantity = get_backordered_quantity(available_quantity, new_on_hand_quantity)
-      unstock_quantity = desired_shipment.stock_location.backorderable?(variant) ? quantity : new_on_hand_quantity
+
+      # Determine how many backordered and on_hand items we'll need to move. We
+      # don't want to move more than what's being asked. And we can't move a
+      # negative amount, which is why we need to perform our min/max logic here.
+      backordered_quantity_to_move = [backordered_quantity, quantity].min
+      on_hand_quantity_to_move = [quantity - backordered_quantity_to_move, 0].max
 
       ActiveRecord::Base.transaction do
         if handle_stock_counts?
           # We only run this query if we need it.
           current_on_hand_quantity = [current_shipment.inventory_units.pre_shipment.size, quantity].min
+          unstock_quantity = desired_shipment.stock_location.backorderable?(variant) ? quantity : new_on_hand_quantity
 
           # Restock things we will not fulfil from the current shipment anymore
           current_stock_location.restock(variant, current_on_hand_quantity, current_shipment)
@@ -111,14 +117,14 @@ def run_tracking_inventory
           inventory_units.
           where(variant:).
           order(state: :asc).
-          limit(backordered_quantity).
+          limit(backordered_quantity_to_move).
           update_all(shipment_id: desired_shipment.id, state: :backordered)
 
         current_shipment.
           inventory_units.
           where(variant:).
           order(state: :asc).
-          limit(quantity - backordered_quantity).
+          limit(on_hand_quantity_to_move).
           update_all(shipment_id: desired_shipment.id, state: :on_hand)
       end
     end

core/spec/models/spree/fulfilment_changer_spec.rb

@@ -153,6 +153,29 @@
 
     it_behaves_like "moves inventory units between shipments"
     it_behaves_like "properly manages inventory units"
+
+    context "and we're only moving some backordered units" do
+      let(:stock_item) { variant.stock_items.find_by!(stock_location: current_shipment.stock_location) }
+      let(:current_shipment_inventory_unit_count) { 10 }
+      let(:quantity) { 4 }
+
+      before do
+        current_shipment.inventory_units.limit(6).update!(state: :backordered)
+        stock_item.update_column(:count_on_hand, -6)
+      end
+
+      it_behaves_like "moves inventory units between shipments"
+
+      it "only removes the desired quantity of backordered items" do
+        subject
+
+        expect(current_shipment.inventory_units.on_hand.count).to eq(4)
+        expect(current_shipment.inventory_units.backordered.count).to eq(2)
+
+        expect(desired_shipment.inventory_units.on_hand.count).to eq(0)
+        expect(desired_shipment.inventory_units.backordered.count).to eq(4)
+      end
+    end
   end
 
   context "when tracking inventory is not set (same as false)" do