title: sqlalchemy update part of date
body: <p>I have a table with <code>Date</code> and I need to set only <code>day</code> and <code>month</code> part of date to <code>1</code> and keep the original year for a set of ids.</p>
  
  <p>So I was trying to form expression like (and variations of it):</p>
  
  <pre><code>session.query(
      UserData
  ).filter(
      UserData.id.in_(selected_ids)
  ).update(
      {
          'user_date': cast((func.date_part('year', UserData.user_date), 1, 1), Date),
      },
      synchronize_session='fetch',
  )
  </code></pre>
  
  <p>I'm simply unable to get it right.
  Help?</p>