In our applications, many of our controllers should only be interacted with via AJAX. This is easy enough to enforce in a controller, using the special verification filter:

class PostController < ApplicationController
verify :xhr => true, :render => { :text => "This action must be accessed using XMLHttpRequest." }
end

However, it took me a little while to figure out how to test this appropriately. The following functional test for the above controller will fail:

def test_edit
post :edit, :id => 1
assert_kind_of Post, assigns("post")
end

In order to simulate an AJAX request, you need to use the xhr method instead of the post method:

def test_edit
xhr :post, :edit, :id => 1
assert_kind_of Post, assigns("post")
end

Unfortunately, this technique doesn’t seem to be mentioned in the Guide to Testing the Rails.