Multiple user models with Ruby On Rails and devise to have separate registration but one common login

i have same problem just like this link:

Multiple user models with Ruby On Rails and devise to have separate registration routes but one common login route

in my app there are two model

1) Company

2) Employee

my User model

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
     :recoverable, :rememberable, :trackable, :validatable

  belongs_to :usr, :polymorphic => true
end

my Company model

class Company < ActiveRecord::Base
  has_many :users, :as => :usr
end

my Employee model

class Employee < ActiveRecord::Base
  has_many :users, :as => :usr
end

app/views/devise/registrations/new.html.erb

<h2>Sign up</h2>

<%
  # customized code begin

  params[:user][:user_type] ||= 'company'

  if ["company", "employee"].include? params[:user][:user_type].downcase
    child_class_name = params[:user][:user_type].downcase.camelize
    user_type = params[:user][:user_type].downcase
  else
    child_class_name = "Company"
    user_type = "company"
  end

  resource.usr = child_class_name.constantize.new if resource.usr.nil?

  # customized code end
%>



<%= form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
  <%= devise_error_messages! %>

  <div class="field">
    <%= f.label :email %><br />
    <%= f.email_field :email, autofocus: true %>
  </div>

  <div class="field">
    <%= f.label :password %>
    <% if @validatable %>
    <em>(<%= @minimum_password_length %> characters minimum)</em>
    <% end %><br />
    <%= f.password_field :password, autocomplete: "off" %>
  </div>

  <div class="field">
    <%= f.label :password_confirmation %><br />
    <%= f.password_field :password_confirmation, autocomplete: "off" %>
  </div>


  <% # customized code begin %>
  <%= fields_for resource.usr do |rf| %>
    <% render :partial => "#{child_class_name.underscore}_fields", :locals => { :f => rf } %>
  <% end %>

  <%= hidden_field :user, :user_type, :value => user_type %>
  <% # customized code end %>



  <div class="actions">
    <%= f.submit "Sign up" %>
  </div>
<% end %>

<%= render "devise/shared/links" %>

my registration create method :

controllers/users/registrations_controller.rb

class Users::RegistrationsController < Devise::RegistrationsController
 def create
    build_resource

    # customized code begin

    # crate a new child instance depending on the given user type
    child_class = params[:user][:user_type].camelize.constantize
    resource.usr = child_class.new(params[child_class.to_s.underscore.to_sym])

    # first check if child instance is valid
    # cause if so and the parent instance is valid as well
    # it's all being saved at once
    valid = resource.valid?
    valid = resource.usr.valid? && valid

    # customized code end

    if valid && resource.save    # customized code
      if resource.active_for_authentication?
        set_flash_message :notice, :signed_up if is_navigational_format?
        sign_in(resource_name, resource)
        respond_with resource, :location => redirect_location(resource_name, resource)
      else
        set_flash_message :notice, :inactive_signed_up, :reason => inactive_reason(resource) if is_navigational_format?
        expire_session_data_after_sign_in!
        respond_with resource, :location => after_inactive_sign_up_path_for(resource)
      end
    else
      clean_up_passwords(resource)
      respond_with_navigational(resource) { render :new }
    end
  end
end

my application helper file :

module ApplicationHelper
    def my_devise_error_messages!
    return "" if resource.errors.empty? && resource.usr.errors.empty?

    messages = usr_messages = ""

    if !resource.errors.empty?
      messages = resource.errors.full_messages.map { |msg| content_tag(:li, msg) }.join
    end

    if !resource.usr.errors.empty?
      usr_messages = resource.usr.errors.full_messages.map { |msg| content_tag(:li, msg) }.join
    end

    messages = messages + usr_messages   
    sentence = I18n.t("errors.messages.not_saved",
                      :count => resource.errors.count + resource.usr.errors.count,
                      :resource => resource.class.model_name.human.downcase)

    html = <<-HTML
    <div id="error_explanation">
    <h2>#{sentence}</h2>
    <ul>#{messages}</ul>
    </div>
    HTML

    html.html_safe
  end
end

but i got error something like this when i open any user sign_up page:

localhost:3000/emloyees/sign_up **OR**

localhost:3000/companies/sign_up

error in registration view

ERROR :-

so help me what i do wrong..???

thanks..!!