Modify Spree::Stock::Quantifier to not re-fetch stock items if they are already eager-loaded

This helps to remove a big N+1 here, and will also be very helpful elsewhere in the app

app/models/spree/stock/quantifier.rb

@@ -5,7 +5,7 @@ module Spree
 
       def initialize(variant)
         @variant = variant
-        @stock_items = Spree::StockItem.joins(:stock_location).where(:variant_id => @variant, Spree::StockLocation.table_name =>{ :active => true})
+        @stock_items = fetch_stock_items
       end
 
       def total_on_hand
@@ -23,6 +23,16 @@ module Spree
       def can_supply?(required)
         total_on_hand >= required || backorderable?
       end
+
+      private
+
+      def fetch_stock_items
+        # Don't re-fetch associated stock items from the DB if we've already eager-loaded them
+        return @variant.stock_items.to_a if @variant.stock_items.loaded?
+
+        Spree::StockItem.joins(:stock_location).
+          where(:variant_id => @variant, Spree::StockLocation.table_name => { active: true })
+      end
     end
   end
 end