user.rb

class User < ActiveRecord::Base

  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  attr_accessible :email, :password, :password_confirmation, :remember_me

  belongs_to :account, :polymorphic => true

  def realstater?
    account.kind_of? Realstater
  end
end

user_spec.rb

require 'spec_helper'

describe User do
  context 'when I have a user with realstater account' do
    let(:realstater)  { create :realstater }
    subject           { create :user, :account => realstater }

    its(:realstater?) { should be_true }
  end
end