title: SQLAlchemy func issue with date and .isocalendar()
body: <p>I have a table that stores tasks submitted by users, with timestamps. I would like to write a query that returns certain rows based on when they were submitted (was it this day/week/month..). </p>
  
  <p>To check if it was submitted on this week, I wanted to use <code>date.isocalendar()[1]</code> function. The problem is, that my timestamps are <code>datetime</code>s, so I would need to transform those to <code>date</code>s.</p>
  
  <p>Using func:
  <code>filter(func.date(Task.timestamp) == datetime.date(datetime.utcnow()))</code>
  works properly. </p>
  
  <p>But I need the <code>date</code> object's <code>isocalendar()</code> method, so I try</p>
  
  <p><code>filter(func.date(Task.timestamp).isocalendar()[1]==datetime.date(datetime.utcnow()).isocalendar()[1])</code></p>
  
  <p>and it's no good, I get <code>AttributeError: Neither 'Function' object nor 'Comparator' object has an attribute 'isocalendar'</code></p>
  
  <p>If I make a simple query and try <code>datetime.date(task.timestamp).isocalendar()[1]</code> it works properly. </p>
  
  <p>How do I get it to work in the query's filter?</p>