title: How to alter a postgres datetime column to default to datetime.utcnow
body: <p>I'm working with flask(sqlalchemy) and postgres, i already declared a model with a datetime column that defaults to <code>datetime.utcnow()</code> courtesy of the datetime module in python. however i noticed on new row insertions the time never changes, i did a few digging and found i shouldn't be calling the function but rather passing it thus: <code>datetime.utcnow</code></p>
  <p>So, i now wish to alter the column to reflect this change without having to drop the table/column.</p>
  <p>I already tried <code>ALTER TABLE mytable ALTER COLUMN trans_time SET DEFAULT datetime.utcnow</code> and i get the following error: <code>ERROR:  cannot use column reference in DEFAULT expression</code></p>
  <p>Note: I don't have migrations set up for this project so that would not help for now. i only need to do this via sql commands.</p>