gistfile1.rb

# post.rb
class Post
  include DataMapper::Resource

  property :id,    Serial
  property :title, String, :required => true
  property :body,  Text,   :required => true
  property :slug,  String

  before :save, :make_slug

  def make_slug
    self.slug = title.downcase.gsub(/\W/,'-').squeeze('-').chomp('-')
  end
end

# post_spec.rb
it "should make a slug from the title" do
  post_with_generated_slug = Factory(:post, title: "Star Wars")
  post_with_generated_slug.slug.should == "star-wars"
end