Rails API only with Okta but without Devise

application.rb

require_relative 'boot'

require 'rails'
# Pick the frameworks you want:
require 'active_model/railtie'
require 'active_job/railtie'
require 'active_record/railtie'
require 'action_controller/railtie'
require 'action_mailer/railtie'
require 'action_view/railtie'

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(*Rails.groups)

module APP_Api
  class Application < Rails::Application
    # Initialize configuration defaults for originally generated Rails version.
    config.load_defaults 6.0

    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration can go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded after loading
    # the framework and any gems in your application.

    # Only loads a smaller set of middleware suitable for API only apps.
    # Middleware like session, flash, cookies can be added back manually.
    # Skip views, helpers and assets when generating a new resource.
    config.api_only = true

    # https://github.com/omniauth/omniauth#integrating-omniauth-into-your-rails-api
    config.session_store :cookie_store, key: '_APP_session', expire_after: 1.day
    config.middleware.use ActionDispatch::Cookies
    config.middleware.use ActionDispatch::Session::CookieStore, config.session_options

    # Use :sql format instead of default :ruby in order to support PostgreSQL specific features
    # such as views. See https://guides.rubyonrails.org/active_record_migrations.html#types-of-schema-dumps.
    config.active_record.schema_format = :sql
  end
end

application_controller.rb

class ApplicationController < ActionController::API
   include ActionController::Caching
  
   before_action :authenticate_user!
  
  def authenticate_user!
    jwt =JwtAuthService.decode(request_token)
    user_id = jwt['user_id']
    @current_user = User.find(user_id)
  rescue JWT::VerificationError, JWT::DecodeError, ActiveRecord::RecordNotFound
    raise Exception.new(status: 401)
  end

auth_controller.rb

# frozen_string_literal: true

class AuthenticationController < ApplicationController
  include ActionController::RequestForgeryProtection
  include ActionController::Cookies
  include AbstractController::Rendering
  include ActionView::Layouts

  skip_before_action :authenticate_user!

  # SECURITY: Must not be accessible across origins (from untrusted domains)
  def csrf_token
    render json: { csrfToken: form_authenticity_token }
  end

  def callback
    user = User.create_or_initialize_by(email: request.env['omniauth.auth'].dig('info', 'email')&.downcase)
    if user
      session[:okta] = { user_id: user.id, expires: 30.seconds.since }
      redirect_to '/login?submit=1'
    else
      raise Exception.new(status: 401, title: 'User is not authorized')
    end
  end

  def create
    user =
      if session[:okta] && session[:okta].fetch(:expires, 0) > Time.zone.now
        User.find_by(id: session[:okta][:user_id])
      end

    if user
      render json: { jwt: JwtAuthService.encode({ user_id: user.id }) }
    else
      raise Exception.new(status: 401, title: 'No active session found')
    end
  end

  private

  def auth
    request.env['omniauth.auth']
  end
 
end

Gemfile

source 'https://rubygems.org'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '~> 6.0.0'

# Shorten boot time
gem 'bootsnap'

# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', github: 'rails/jbuilder'
gem 'oj'

# Authentication
gem 'jwt'
gem 'omniauth-okta', github: 'dandrews/omniauth-okta'
gem 'omniauth-rails_csrf_protection'

# PostgreSQL
gem 'pg'

JwtAuthService.rb

require 'jwt'
  def encode(payload)
    payload = payload.dup
    payload['exp'] = tomorrow.to_i
    JWT.encode(payload, Rails.application.secrets.secret_key_base)
  end
  
   def decode(token)
    decoded_token = JWT.decode(token, Rails.application.secrets.secret_key_base)
    decoded_token.first
 end

login.page.html

<div class="login"
  [ngClass]="{'loading': isLoading}">
  <form #oktaForm ngNoForm action="/auth/okta" method="POST" (submit)="isLoading=true">
    <input type="hidden" name="authenticity_token" [value]="csrfToken">
    <button class="login__button"
      [disabled]="!csrfToken"
      (click)="onSubmit(oktaForm)">
      Login with Okta
    </button>
  </form>
</div>

login.page.ts

class LoginPage {
  csrfToken: string
  isLoading = false

  login() {
  # Pseudocode
    getCsrfToken(): Observable<{csrfToken: string}> { 
      return this.api.request('GET', '/auth/csrf_token')
    }
  }
  

omniauth.rb

return if (Rails.env.test? || Rails.env.e2e?)

require 'omniauth-okta'

Rails.application.config.middleware.use(
  OmniAuth::Strategies::Okta,
  ENV.fetch('OKTA_CLIENT_ID'),
  ENV.fetch('OKTA_CLIENT_SECRET'),
  scope: 'email groups',
  fields: %w[email groups],
  client_options: {
    site: ENV.fetch('OKTA_ISSUER'),
    authorize_url: ENV.fetch('OKTA_ISSUER') + '/v1/authorize',
    token_url: ENV.fetch('OKTA_ISSUER') + '/v1/token',
    user_info_url: ENV.fetch('OKTA_ISSUER') + '/v1/userinfo',
    redirect_uri: ENV.fetch('OKTA_REDIRECT_URI')
  }
)

routes.rb

get '/auth/csrf_token', to: 'authentication#csrf_token'
get '/auth/okta/callback', to: 'authentication#callback'

Very helpful! Thanks