title: Rails migrate add column and populate with data from another column
body: <p>If I change my mind about how I store Model data, how might I add new columns and migrate existing data over?</p>
  
  <p>For example, would the following be appropriate:</p>
  
  <pre><code>class AddPropertyAAndPropertyBToOwner &lt; ActiveRecord::Migration
    def change
      add_column :owners, :property_a, :string
      add_column :owners, :property_b, :string
  
      # Now I need to migrate the existing 40 records
      # populating the new columns from an existing one
      # Owner.all.each do |o|
      #   original = o.original_property
      #   o.property_a = original.match(/foo/).captures.first
      #   o.proptery_b = original.match(/bar/).captures.first
      #   o.save
      # end
    end
  end
  </code></pre>