Forward all posts through proxy to mongrels

I’m using caching in rails, as well as restful routes. My issue is that
the url for the save action looks exactly like the url for the show
action, so attempts to save get sent by nginx directly to the cache
instead of going to the mongrels on the back end.

I’ve tried to fix this by adding a conditional proxy pass that should
send all posts to the mongrel, but for some reason it is being ignored.

Here’s my config:

Can anyone see what is wrong with it?