From 7487b9a830f960ec846cd3e13de063cb818db2da Mon Sep 17 00:00:00 2001
From: Jared Norman <jared@super.gd>
Date: Thu, 15 Oct 2026 12:33:52 -0700
Subject: [PATCH] Handle different line item/inventory unit variants

When a line item had inventory units, the validator counted pending
units per stock location and checked line_item.variant against each
count. This assumed every unit shared the line item's variant.

This assumption doesn't hold when stores use solidus_product_assembly or
implement their own bundling systems. It also doesn't hold even without
such customizations: `ReturnItem#build_exchange_inventory_unit` attaches
a unit with the exchange variant to the original line item. This means
that an exchange on an order that gets canceled and resumed (admittedly
not a common thing) would get validated against the original variant's
stock, not the exchanged variant. Technically a bug.

The `InventoryUnitsFinalizer` was fixed ages ago to unstock each unit's
own variant. This change updates the validator to check stock the same
way, against the inventory unit's variant.

For ordinary stores, this doesn't change anything. For stores that
implement bundling or use solidus_product_assembly, we can simplify
things. Previously Product Assembly replaced this whole class; after
this it'll only have to make one small modification.

References:

- Discussion #4489
- The finalizer was fixed in d3411e6b
- https://jardo.dev/an-update-on-bundles-in-solidus
---
 .../spree/stock/availability_validator.rb     | 10 ++++--
 .../stock/availability_validator_spec.rb      | 36 +++++++++++++++++++
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/core/app/models/spree/stock/availability_validator.rb b/core/app/models/spree/stock/availability_validator.rb
index dbf05cc1bf0..31b6f6ba219 100644
--- a/core/app/models/spree/stock/availability_validator.rb
+++ b/core/app/models/spree/stock/availability_validator.rb
@@ -25,9 +25,13 @@ def is_valid?(line_item)
         if line_item.inventory_units.empty?
           Spree::Config.stock.quantifier_class.new(line_item.variant).can_supply?(line_item.quantity)
         else
-          quantity_by_stock_location_id = line_item.inventory_units.pending.joins(:shipment).group(:stock_location_id).count
-          quantity_by_stock_location_id.all? do |stock_location_id, quantity|
-            Spree::Config.stock.quantifier_class.new(line_item.variant, stock_location_id).can_supply?(quantity)
+          quantity_by_variant_and_stock_location = line_item.inventory_units.pending.joins(:shipment)
+            .group(:variant_id, :stock_location_id).count
+          variant_ids = quantity_by_variant_and_stock_location.keys.map(&:first)
+          variants = Spree::Variant.with_discarded.where(id: variant_ids).index_by(&:id)
+
+          quantity_by_variant_and_stock_location.all? do |(variant_id, stock_location_id), quantity|
+            Spree::Config.stock.quantifier_class.new(variants.fetch(variant_id), stock_location_id).can_supply?(quantity)
           end
         end
       end
diff --git a/core/spec/models/spree/stock/availability_validator_spec.rb b/core/spec/models/spree/stock/availability_validator_spec.rb
index f74262005e8..76ccc95d3aa 100644
--- a/core/spec/models/spree/stock/availability_validator_spec.rb
+++ b/core/spec/models/spree/stock/availability_validator_spec.rb
@@ -106,6 +106,42 @@ module Stock
             include_examples "passes validation"
           end
         end
+
+        context "when an inventory unit has a different variant than the line item" do
+          let(:line_item) { order.line_items.first }
+          let(:other_variant) { create(:variant) }
+
+          before do
+            create(:inventory_unit, variant: other_variant, line_item:, shipment: order.shipments.first)
+            line_item.variant.stock_items.update_all(count_on_hand: 10, backorderable: false)
+          end
+
+          context "and the other variant has no stock" do
+            before do
+              other_variant.stock_items.update_all(count_on_hand: 0, backorderable: false)
+            end
+
+            include_examples "fails validation"
+          end
+
+          context "and the other variant has stock" do
+            before do
+              other_variant.stock_items.update_all(count_on_hand: 1, backorderable: false)
+            end
+
+            include_examples "passes validation"
+          end
+
+          context "and only the other variant's units are pending" do
+            before do
+              line_item.inventory_units.where(variant: line_item.variant).update_all(pending: false)
+              line_item.variant.stock_items.update_all(count_on_hand: 0, backorderable: false)
+              other_variant.stock_items.update_all(count_on_hand: 1, backorderable: false)
+            end
+
+            include_examples "passes validation"
+          end
+        end
       end
 
       context "line_item is split across two shipments" do