ajax replacing partial not working

You don't need the underscore when referencing a partial this way.

Change: page.replace_html 'comments', :partial => '_comments', :object => bar

To this: page.replace_html 'comments', :partial => 'comments', :object => bar

I had this problem for awhile and it was because of the underscore. You might want to specify the controller when calling that partial and see if that works.

:partial => 'controllerName/comments'