title: Update one column to value of another in Rails migration
body: <p>I have a table in a Rails app with hundreds of thousands of records, and they only have a <code>created_at</code> timestamp. I'm adding the ability to edit these records, so I want to add an <code>updated_at</code> timestamp to the table. In my migration to add the column, I want to update all rows to have the new <code>updated_at</code> match the old <code>created_at</code>, since that's the default for newly created rows in Rails. I could do a <code>find(:all)</code> and iterate through the records, but that would take hours because of the size of the table. What I really want to do is:</p>
  
  <pre class="lang-sql prettyprint-override"><code>UPDATE table_name SET updated_at = created_at;
  </code></pre>
  
  <p>Is there a nicer way to do that in a Rails migration using ActiveRecord rather than executing raw SQL?</p>