[Beginning Rails 3] Listing 5-24. validates :uniqueness => true method, in app/models/user.rb

user.rb

class User < ActiveRecord::Base
  validates :email, :uniqueness => true

  has_one :profile
  has_many :articles, :order => 'published_at DESC, title ASC',
                      :dependent => :nullify
  has_many :replies, :through => :articles, :source => :comments
end