title: Add column to table and fix value for existing records in Rails
body: <p>I want to add a column called "payment_type" into my "orders" table.</p>
  
  <p>Here is the migration that I have so far:</p>
  
  <pre><code>def change
    add_column :orders, :payment_type, :string
  end
  </code></pre>
  
  <p>I want that payment_type to hold the value "normal" for all the records that currently are in the DB. However, not for the future records. I want no default value for future records. How can I do this?</p>