PERF: don't ever use inject on AR relations

inject will deteriorate to a method missing that
invokes arel and a world of uneeded work

app/serializers/category_detailed_serializer.rb

@@ -72,7 +72,11 @@ class CategoryDetailedSerializer < BasicCategorySerializer
   end
 
   def count_with_subcategories(method)
-    object.subcategories.inject(object.send(method) || 0) { |sum,c| sum += (c.send(method) || 0) }
+    count = object.send(method) || 0
+    object.subcategories.each do |category|
+      count += (object.send(method) || 0)
+    end
+    count
   end
 
 end