Rspec: Anonymous controller

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery
  rescue_from ActiveRecord::RecordNotFound, :with => :not_found

  private
  def not_found
    respond_to do |format|
     format.html { render file: "#{Rails.root}/public/404", status: 404, layout: false}
    end
    true
  end
  
end

application_controller_spec.rb

require 'spec_helper'

describe ApplicationController do

  context "when record not found" do
    controller do
      def index
        raise ActiveRecord::RecordNotFound
      end
    end

    it "redirects to 404" do
      get :index
      expect(response).to render_template(file: "#{Rails.root}/public/404")
    end
  end

end