Testing ApplicationController before_filter methods using RSpec's "anonymous" controller instance

application_controller.rb

class ApplicationController < ActionControllerBase
  helper :do_something

  def do_something
    @from_do_something = params[:for_do_something]
  end
end

application_controller_spec.rb

require 'rails_helper'

describe ApplicationController do
  controller do
    before_filter :do_something

    def action_requiring_filter
      render nothing: true
    end
  end

  before(:each) do
    routes.draw do
      get 'action_requiring_filter' => 'anonymous#action_requiring_filter'
    end
  end

  context "when an action requires a before filter" do

    it "runs the filter" do
      get :action_requiring_filter, for_do_something: 'some value'

      expect(assigns[:from_do_something]).to eq('some value')
    end
  end
end

This is not to suggest that it's a good idea to have before filters that set instance variables, but if you do need to test them, this technique at least makes the controller behave more like the real world. Too often I see tests that directly call the filter method on the ApplicationController, which can work in certain circumstances, but normally requires a bunch of stubbing of params and environment details because the method is being called outside of the normal Rails request/response cycle.

This also demonstrates a way to test any actions that depend on "inherited" behavior from ApplicationController, using RSpec's anonymous controller context and some test routes.