diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 62cc414555a90442ce3f0c406ae2bdb4631bcb41..2470eb44b4b56c772735be2ffa425d45612a17fe 100755
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -1307,7 +1307,7 @@ def collection_reader_method(reflection, association_proxy_class)
             if send(reflection.name).loaded?
               send(reflection.name).map(&:id)
             else
-              send(reflection.name).all(:select => "#{reflection.quoted_table_name}.id").map(&:id)
+              send(reflection.name).all(:select => "#{reflection.quoted_table_name}.#{reflection.klass.primary_key}").map(&:id)
             end
           end
         end