Permit parameters for nested forms in devise registration

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 :loginable, polymorphic: true, validate: true
  accepts_nested_attributes_for :loginable
end

class Professor < ActiveRecord::Base
  has_and_belongs_to_many :colleges
  has_one :user, as: :loginable, dependent: :destroy
end

class Student < ActiveRecord::Base
  belongs_to :college
  has_one :user, as: :loginable, dependent: :destroy
end

<h2>Sign up</h2>

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

    <div>
      <%= f.label :first_name %><br />
      <%= f.text_field :first_name, autofocus: true %>
    </div>

    <div>
      <%= f.label :middle_name %><br />
      <%= f.text_field :middle_name %>
    </div>

    <div>
      <%= f.label :last_name %><br />
      <%= f.text_field :last_name %>
    </div>

    <div>
      <%= f.label :email %><br />
      <%= f.email_field :email %>
    </div>

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

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

    <div id="select_type">
      <%= f.label :user_type %><br />
      <%= f.select :loginable_type, options_for_select(%w[Professor Student]) %>
    </div>

    <div id="student_reg">
      <%= f.fields_for Student.new do |rf| %>
          <%= render :partial => "students/form", :locals => { :f => rf } %>
      <% end %>
    </div>

    <div id="prof_reg">
      <%= f.fields_for Professor.new do |rf| %>
          <%= render :partial => "professors/form", :locals => { :f => rf } %>
      <% end %>
    </div>

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

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

<%= f.label :registration_number %><br />
<%= f.text_field :reg_number %>

<%= f.label :degree %><br />
<%= f.text_field :degree %>

class RegistrationsController < Devise::RegistrationsController

  def create
    build_resource(sign_up_params)

    if resource.loginable_type == 'Professor'
      resource.loginable = Professor.new
    else
      resource.loginable = Student.new
    end

    resource_saved = resource.save
    yield resource if block_given?
    if resource_saved
      if resource.active_for_authentication?
        set_flash_message :notice, :signed_up if is_flashing_format?
        sign_up(resource_name, resource)
        respond_with resource, location: after_sign_up_path_for(resource)
      else
        set_flash_message :notice, :"signed_up_but_#{resource.inactive_message}" if is_flashing_format?
        expire_data_after_sign_in!
        respond_with resource, location: after_inactive_sign_up_path_for(resource)
      end
    else
      clean_up_passwords resource
      @validatable = devise_mapping.validatable?
      if @validatable
        @minimum_password_length = resource_class.password_length.min
      end
      respond_with resource
    end
  end

  def destroy
    resource.loginable.destroy
    Devise.sign_out_all_scopes ? sign_out : sign_out(resource_name)
    set_flash_message :notice, :destroyed if is_flashing_format?
    yield resource if block_given?
    respond_with_navigational(resource){ redirect_to after_sign_out_path_for(resource_name) }
  end

  private

  def sign_up_params
    params.require(:user).permit(:first_name, :middle_name, :last_name, :email, :password, :password_confirmation, :loginable_type, :student => [:reg_number], :professor => [:degree])
  end

  def account_update_params
    params.require(:user).permit(:first_name, :middle_name, :last_name, :email, :password, :password_confirmation, :current_password)
  end
end

before_filter :update_sanitized_params, if: :devise_controller?

def update_sanitized_params
    devise_parameter_sanitizer.for(:sign_up) {|u| u.permit(:first_name, :middle_name, :last_name, :email, :password, :password_confirmation, :loginable_type, student: [:reg_number], professor: [:degree])}
  end