application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
  helper_method :current_user

  def authenticate
  	redirect_to :login unless user_signed_in?
  end

  def current_user

  	@current_user ||= User.find(session[:user_id]) if session[:user_id]

    # @current_user || @current_user = User.find(session[:user_id]) if session[:user_id]
  end

  def user_signed_in?
  	# converts current_user to a boolean by negating the negation
  	!!current_user
  end

end

groups_controller.rb

class GroupsController < ApplicationController
	
	before_action :authenticate
	
  # clipped for brevity
end

groups_controller_spec.rb

require 'rails_helper'

RSpec.describe GroupsController, type: :controller do

	describe 'GET #show' do
		
		context 'when user logged in' do

			before do
				Rails.application.env_config["omniauth.auth"] = OmniAuth.config.mock_auth[:google_oauth2]
			end

			it "responds with HTTP 200 and a group object" do
				get :show, params: {id: 1}, session: {user_id: 1} # ADDING SESSION HASH WAS THE FIX!
				expect(response).to have_http_status(200)
			end

		end

		context 'when user logged out' do

		end

	end
end

routes.rb

Rails.application.routes.draw do

	get 'login', to: redirect('/auth/google_oauth2'), as: 'login'
	get 'logout', to: 'sessions#destroy', as: 'logout'
	get 'auth/:provider/callback', to: 'sessions#create'
	get 'auth/failure', to: redirect('/')

	get '/users/:id/groups', to: 'users#groups'

	resources :groups, only: [:show, :create]

end

test.rb

# config/environments/test.rb

# Mock user account for OmniAuth integration testing
  OmniAuth.config.test_mode = true
  OmniAuth.config.mock_auth[:google_oauth2] = OmniAuth::AuthHash.new({
    'provider': 'google_oauth2',
    'uid': '1',
    # etc.
  })