page.rb

class Page
  include Mongoid::Document
  
  field :title
  field :body
  field :permalink
  
  validates_presence_of :title, :body
  before_validation :generate_permalink
  
  attr_accessible :title, :body
  
  protected
  def generate_permalink    
    self.permalink = title.to_permalink
  end
end

page_spec.rb

require 'spec_helper'

describe Page do
  describe "permalink"
    it "must derivate from title" do
      subject.title = subject.body = "Testing the Permalink"
      subject.save!
      subject.permalink.should == "testing-the-permalink"
    end
  end
end