title: Add default value to a migration of another column
body: <p>Since the updated_at column looks at all attributes when updated I want to add a new_updated_at which I will decide when it updates.</p>
  
  <p>The default value on insertion via migration I want it to be the current updated_at.</p>
  
  <p>How do I set the default value of the migration to be the value of the same row but of another column?</p>