title: Rails: setting column default to created_at timestamp
body: <p>I am using a Postgres DB with Rails v4.1.6</p>
  
  <p>I have a table with a <code>start_time</code> column. </p>
  
  <p>I want the <code>start_time</code> column to be non null and have a default value of the timestamp that will be the value in <code>created_by</code>.</p>
  
  <p>My understanding is that the timestamp value for <code>created_by</code> (and <code>updated_at</code>) are created by Rails and not enforced by the DB (please let me know if this is wrong). If there a way to match these two columns up? Is there a way to have the DB enforce it?</p>
  
  <p>My only solution right now is to use a before_create filter (but I think there would be inconsistencies between the two values).</p>