Use reflection primary_key instead of id for when selecting association ids.

[#906 state:resolved]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

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