https://traversal.ist/how-to-authenticate-anonymous-users-in-rails

users_controller.rb

class UsersController < ApplicationController

  skip_before_action :authenticate_request, only: [:anon]

  # create/register a user
  def create
    # transition anonymous account to full account
    updateStatus = @current_user.update(email: params[:email], display_name: params[:display_name], password: params[:password])
    if updateStatus
      # delete the associated anonymous user
      @current_user.anonymous_user.destroy
      render json: {status: 'User created successfully'}, status: :created
    else
      render json: {errors: @current_user.errors.full_messages}, status: :conflict
    end
  end

  # login a user
  def login
    if @current_user.anonymous_user
      # TODO: Stretch Goal: associate anonymous User’s actions around the site
      # 	with the already-existing user they’re logging in as. This will involve
      # 	any records in the database for the anonymous user to the already-existing
      #	user's ID, and recording some kind of record of this for any external
      #	services that involve user_id’s and can’t be updated.
    end

    # get a user by the email supplied
    user = User.find_by(email: params[:email].to_s.downcase)
    # check that the user exists and the password is correct
    if user && user.authenticate(params[:password])
      render json: {auth_token: JsonWebToken.encode({user_id: user.id})}, status: :ok
    else
      render json: {error: 'Invalid email or password'}, status: :unauthorized
    end
  end

  # create and login an anonymous user
  def anon
    new_anon_user = User.create
    AnonymousUser.create(display_name: AnonymousUser.random_name, user: new_anon_user)
    render json: {auth_token: JsonWebToken.encode({user_id: new_anon_user.id})}, status: :ok
  end

  def current
    user = @current_user
    render json: {
      email: user.email,
      display_name: user.display_name,
      anon_display_name: user.anon_display_name
    }, status: :ok
  end

end