Move increment/decrement stock level to dedicated service

rails_application/app/controllers/orders_controller.rb

@@ -77,7 +77,7 @@ def add_item
 
     @order = Order.find(params[:id])
     @order.add_item(product)
-    product.decrement!(:stock_level)
+    Inventory::ProductService.new.decrement_stock_level(product.id)
     @order.save!
 
     redirect_to edit_order_path(params[:id])
@@ -87,7 +87,7 @@ def remove_item
     product = Product.find(params[:product_id])
     @order = Order.find(params[:id])
     @order.remove_item(product)
-    product.increment!(:stock_level)
+    Inventory::ProductService.new.increment_stock_level(product.id)
     @order.save!
 
     redirect_to edit_order_path(params[:id])
@@ -113,7 +113,7 @@ def expire
       .where(status: "Draft")
       .find_each do |order|
       order.order_items.each do |item|
-        item.product.increment!(:stock_level)
+        Inventory::ProductService.new.increment_stock_level(item.product.id)
       end
       order.status = "Expired"
       order.save!

rails_application/app/models/inventory/product_service.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module Inventory
+  class ProductService
+
+    def decrement_stock_level(product_id)
+      product = ::Product.find(product_id)
+      product.decrement!(:stock_level)
+    end
+
+    def increment_stock_level(product_id)
+      product = ::Product.find(product_id)
+      product.increment!(:stock_level)
+    end
+  end
+end