refactor to use group_by

activerecord/lib/active_record/association_preload.rb

@@ -336,11 +336,11 @@ def preload_belongs_to_association(records, reflection, preload_options={})
             end
           end
         else
-          id_map = {}
-          records.each do |record|
+          id_map = records.group_by do |record|
             key = record.send(primary_key_name)
-            (id_map[key.to_s] ||= []) << record if key
+            key && key.to_s
           end
+          id_map.delete nil
           klasses_and_ids[reflection.klass.name] = id_map unless id_map.empty?
         end