Autoremove/update item from the cart if stock changed during checkout

app/controllers/concerns/order_stock_check.rb

@@ -11,7 +11,8 @@ def valid_order_line_items?
   end
 
   def handle_insufficient_stock
-    return if sufficient_stock?
+    stock_service = Orders::CheckStockService.new(@order)
+    return if stock_service.sufficient_stock?
 
     flash[:error] = Spree.t(:inventory_error_flash_for_insufficient_quantity)
     redirect_to main_app.cart_path
@@ -33,10 +34,4 @@ def check_order_cycle_expiry
       format.html { redirect_to main_app.shop_path, status: :see_other }
     end
   end
-
-  private
-
-  def sufficient_stock?
-    @sufficient_stock ||= @order.insufficient_stock_lines.blank?
-  end
 end

app/services/orders/check_stock_service.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Orders
+  class CheckStockService
+    attr_reader :order
+
+    def initialize(order: )
+      @order = order
+    end
+
+    def sufficient_stock?
+      @sufficient_stock ||= order.insufficient_stock_lines.blank?
+    end
+  end
+end

spec/services/orders/check_stock_service_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Orders::CheckStockService do
+  subject { described_class.new(order:) }
+
+  let(:order) { create(:order_with_line_items) }
+
+  describe "#sufficient_stock?" do
+    it "returns true if enough stock" do
+      expect(subject.sufficient_stock?).to be(true)
+    end
+
+    context "when one or more item are out of stock" do
+      it "returns false" do
+        variant = order.line_items.first.variant
+        variant.update!(on_demand: false, on_hand: 0)
+
+        expect(subject.sufficient_stock?).to be(false)
+      end
+    end
+  end
+end