admin.rb

class Admin < User
  has_one :user, :as => :userable
  attr_accessible :user
end

user.rb

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

  attr_accessible :name, :email, :password, :password_confirmation, :role
  belongs_to :userable, :polymorphic => true

  # Checks if a given user has "role" permission
  def is?(role)
    self.role == role.to_s
  end
end