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'