anonymous_user.rb

class AnonymousUser < User
  attr_accessible *ACCESSIBLE_ATTRS, :type, :token, as: :registrant

  def register(params)
    params = params.merge(type: 'User', token: nil)
    self.update_attributes(params, as: :registrant)
  end
end

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery

  def authenticate_user!(*args)
    current_user.present? || super(*args)
  end

  def current_user
    super || AnonymousUser.find_or_initialize_by_token(anonymous_user_token).tap do |user|
      user.save(validate: false) if user.new_record?
    end
  end

  private
  def anonymous_user_token
    session[:user_token] ||= SecureRandom.hex(8)
  end
end

registrations_controller.rb

class RegistrationsController < Devise::RegistrationsController
   def create
     @user = AnonymousUser.find(current_user.id)
     if @user.register(params[:user])
       sign_in_and_redirect @user.becomes(User)
     else
       render :new
     end
   end
end

routes.rb

RetrospectionApp::Application.routes.draw do
  devise_for :users, controllers: {registrations: 'registrations'}

  resources :posts, path: '/'
  root to: 'posts#index'
end