reuse the same preloader object for each association

activerecord/lib/active_record/relation.rb

@@ -599,9 +599,9 @@ def exec_queries
 
       preload = preload_values
       preload +=  includes_values unless eager_loading?
+      preloader = ActiveRecord::Associations::Preloader.new
       preload.each do |associations|
-        pl = ActiveRecord::Associations::Preloader.new
-        pl.preload @records, associations
+        preloader.preload @records, associations
       end
 
       @records.each { |record| record.readonly! } if readonly_value