OAuth Google controller

oauth_google_controller.rb

class OAuth::GoogleAuthorizationsController < ApplicationController
  CLIENT_ID = Rails.application.credentials.google.client_id
  CLIENT_SECRET = Rails.application.credentials.google.client_secret
  SCOPE = "openid email profile"

  AUTHORIZATION_URL = URI("https://accounts.google.com/o/oauth2/v2/auth")
  TOKEN_URL = URI("https://www.googleapis.com/oauth2/v4/token")
  USER_INFO_URL = URI("https://www.googleapis.com/oauth2/v3/userinfo")

  before_action :validate_state_token, only: :show

  def create
    redirect_to authorization_url.to_s, allow_other_host: true
  end

  def show
    access_token = request_access_token.parse.fetch("access_token")
    user_info = request_user_info(access_token: access_token).parse

    user = User.find_or_create_by(email: user_info.fetch("email"))
    user ||= user_info.fetch("name")

    self.current_user = user
    redirect_to root_url
  end

  private
    def validate_state_token
      state_token = params.fetch(:state)
      unless valid_authenticity_token?(session, state_token)
        raise ActionController::InvalidAuthenticityToken, "The state=#{state_token} token is inauthentic."
      end
    end

    # Generates the OAuth authorization URL that will redirect the user to the OAuth provider.
    # A Rails `form_authenticity_token` is used as the `state` parameter to prevent CSRF. The
    # `callback_url` is where the user is sent back to after authenticating with the OAuth provider.
    def authorization_url
      AUTHORIZATION_URL.build.query(
        client_id: CLIENT_ID,
        redirect_uri: callback_url,
        response_type: "code",
        scope: SCOPE,
        state: form_authenticity_token
      )
    end

    # Requests an OAuth access token from the OAuth provider. The access token is used for subsequent
    # requests to gather information like a users name, email, address, or whatever other information
    # The OAuth provider makes available.
    def request_access_token
      HTTP.post(TOKEN_URL, form: {
        client_id: CLIENT_ID,
        client_secret: CLIENT_SECRET,
        code: params.fetch(:code),
        grant_type: "authorization_code",
        redirect_uri: callback_url
      })
    end

    def request_user_info(access_token:)
      HTTP.auth("Bearer #{access_token}").get(USER_INFO_URL)
    end

    # The URL the OAuth provider will redirect the user back to after authenticating.
    def callback_url
      url_for(action: :show, only_path: false)
    end
end