Writing a controller spec outside of a Rails application

controller_spec.rb

require 'spec_helper'
require 'rails'
require 'action_view'
require 'action_controller'
require 'rspec/rails'

class TestApplication < Rails::Application
end

RSpec.describe 'example', type: :controller do
  controller do
    def index
      head :ok
    end
  end

  it 'works' do
    get :index
    expect(response).to have_http_status(:ok)
  end
end