designer.rb

class Designer < ActiveRecord::Base
  has_one :login, :as => :user, :dependent => :destroy
  accepts_nested_attributes_for :login
end

new.haml

- # VIEW FOR ADDING A NEW DESIGNER
%h1 New Designer
- form_for(@designer) do |f|
	- f.fields_for :login do |login|
		= login.label :email
		%br/
		= login.text_field :email
		%br/
		= login.label :password
		%br/
		= login.password_field :password
		%br/
		= login.label :password_confirmation
		%br/
		= login.password_field :password_confirmation

users.rb

class Users < ActiveRecord::Base
  belongs_to :user, :polymorphic => true
end