title: migration with datamapper
body: <p>I need to add another column (email) to my User model. I do not wish to modify the existing data in MySql database. How should I do that? </p>
  
  <p>Below is code of my original model class, and I am using Sinatra. Thanks in advance.</p>
  
  <pre><code>require 'rubygems'
  require 'data_mapper'
  
  DataMapper.setup(:default,'mysql://root@localhost/database')
  
  class User
    include DataMapper::Resource
    property :id,              Serial
    property :name,            String, :unique=&gt;true, :required=&gt;true
  end
  
  DataMapper.finalize
  </code></pre>