title: In a Rails migraiton, how do I set the default value of a column to be NOW() instead of the time when I ran the migration?
body: <p>I have  a PostGres 9.4 database.  I want to change the default column type of a DATETIME column to be the time when the record was created.  I thought this was the right way, in as far as this is my rails migration</p>
  
  <pre><code>class ChangeDefaultValueForStratumWorkerSubmissions &lt; ActiveRecord::Migration[5.1]
    def change
      change_column_default(:stratum_worker_submissions, :created_at, 'NOW')
    end
  end
  </code></pre>
  
  <p>but when I look at my database, the default timestamp shows as the time when I ran the migration, instead of the expression I want.  How do I write a migration that will do what I want?</p>
  
  <pre><code>      Column       |            Type             |                                 Modifiers
  -------------------+-----------------------------+----------------------------------------------------------------------------
   id                | integer                     | not null default nextval('stratum_worker_submissions_id_seq'::regclass)
   stratum_worker_id | integer                     |
   created_at        | timestamp without time zone | not null default '2018-04-04 19:46:22.781613'::timestamp without time zone
  </code></pre>