Odd nuance on getting erb or js files

I've got a partial rendered in a something.js.rjs file. This works just fine when I call the method on the controller using xhr(etc etc) in my functional tests. So from the point of view of the tests an Ajax request is pulling the right stuff though and the associated assert_selects are finding it in the returned result. But...

When I try to get the page fragment via a button in a remote form then the result is 'could not find template something.html.erb'.

My understanding of this is that the ActionController would look for something.js.rjs if it couldn't find the html.erb. In this case it isn't. The request coming through the remote form seems to look like a standard post request and it's trying to find an html.erb to render the result. Wtf is going on here?

Comments please

John Small