How to structure a ruby class

gistfile1.rb

require 'open-magazine' # requirements

class Article
  include Any::Aweseome::Module                         # includes and extensions
                                                        
  belongs_to :author                                    # relations to other models
                                                        
  attr_accessor :title, :pages                          # attribute definitions

  RESERVED_TITLES = ['Coding Ruby']                     # constants...
                                                        
  attr_accessible :title                                # ActiveRecord mass-assignment white-list and
                                                        # other security related settings

  scope :large, where(:pages.lg => 300)                 # class method macros like finders etc  
  mount_uploader :cover_image, CoverImageUploader

  validates_presence_of :title                          # validations

  # put callbacks & hooks before any methods
  # it's important to be aware of them
  # since they are automagical
  before_save :calculate_price
  after_create :bill_book_stores

  # put class methods first...
  def self.dust_books
    all.each(&:dust!)
  end

  # then instance methods
  # prioritize setter methods above others - they are unexpected
  def title=(str)
    @title = str.strip.downcase
  end

  # other methods
  def nice_title
    title.titleize
  end

  # query methods
  def best_seller?
    Author.awarded.include?(author.name)
  end  
  
  # private & protected methods 
  private
  def bill_book_stores
    Billing.create(self)
  end
end