Helper

module ProductsHelper
  def formatted_price(product)
    number_with_precision
      (product.price_without_taxes/100.0),
      :precision => 2
  end
end

Bloated Model

class Product
  def formatted_price_without_taxes
    Object.new.extend(
        ActionView::Helpers::NumberHelper)
        .number_with_precision(
          (price_without_taxes / 100.0),
            :precision => 2)
    )
  end
end

Exemple

class ArticleDecorator < ApplicationDecorator
  decorates :article
  allows :description

  def title
    article.title.upcase
  end
end

Exemple

article = ArticleDecorator.find(1)
article.title #=> "MON ARTICLE"
article.description #=> "Ma description"
article.created_at #=> NoMethodError

Exemple

class ArticleDecorator < ApplicationDecorator
  def author_name
    article.author.first_name +
      " " +
      article.author.last_name
  end

  def excerpt
    h.truncate article.body
  end
end

ApplicationDecorator

class ApplicationDecorator < Draper::Base
  def created_at
    h.content_tag :span, 
              I18n.l(model.created_at), 
              :class => 'created_at'
  end
end

decorates_association

class Article < ActiveRecord::Base
  belongs_to :author
end

class ArticleDecorator < ApplicationDecorator
  decorates :article
  decorates_association :author
end

class AuthorDecorator < ApplicationDecorator
  decorates :author

  def name
    "#{model.first_name} #{model.last_name}"
  end
end

@article.author.name

Rspec

# spec/decorators/author_decorator_spec.rb
require 'spec_helper'

describe AuthorDecorator do
  context "given a new Author" do
    subject { 
      AuthorDecorator.decorate(
        FactoryGirl.build(:author, 
          :first_name => "Dupond")) 
    }

    it "displays its first_name" do
      subject.first_name.should == "Dupond"
    end
  end
end