title: Ruby on Rails add_column does not add that column to existing rows
body: <p>I have created this migration in Rails:</p>
  
  <pre><code>class AddSocialSecurityNumberToContactTable &lt; ActiveRecord::Migration
    def self.up
      add_column :contacts, :ss_number, :string
    end
  
    def self.down
      remove_column :contacts, :ss_number
    end
  end
  </code></pre>
  
  <p>In my development database, which is a SQLite3 database, all the old Contact records gained the ss_number field, just nil. However, whenever I deploy the code with migrations, the old, pre-migration Contact records do not have a ss_number field. I have restarted the server and logged into the console and verified it both ways on both a production database (MySQL) and a staging database (MySQL).</p>
  
  <p>If, however, I create a new record in the console, it does show a ss_number field for the Contact object.</p>
  
  <pre><code>&gt;&gt; a = Contact.find(1)
  =&gt; #&lt;Contact id: 1, first_name: "Gary", last_name: "Simpson", email: "", home_phone: "", work_phone: "", mobile_phone: "", mailing_street: "123 Main St", mailing_street_2: "", mailing_city: "Little Rock", mailing_state: "AR", mailing_zip: "72205", created_by: 1, created_at: "2010-07-24 19:30:10", updated_at: "2010-07-24 19:30:10"&gt;
  &gt;&gt; a.ss_number
  ActiveRecord::MissingAttributeError: missing attribute: ss_number
   from /srv/www/domain/releases/20100727004056/vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:217:in `send'
   from /srv/www/domain/releases/20100727004056/vendor/rails/activerecord/lib/active_record/associations/association_proxy.rb:217:in `method_missing'
   from (irb):4
  &gt;&gt; a = Contact.new
  =&gt; #&lt;Contact id: nil, first_name: nil, last_name: nil, email: nil, home_phone: nil, work_phone: nil, mobile_phone: nil, mailing_street: nil, mailing_street_2: nil, mailing_city: nil, mailing_state: nil, mailing_zip: nil, created_by: nil, created_at: nil, updated_at: nil, ss_number: nil&gt;
  &gt;&gt; a.ss_number
  =&gt; nil
  </code></pre>
  
  <p>How can I make it so that all existing Contact rows have <code>nil</code> as their value after I migrate?</p>