title: Is there a more succinct for this date checking?
body: <p>Every invoice may have several items and the invoice due date is the date of the earliest due item. Here's what I have but I wonder if there may be a shorter version:</p>
  
  <pre><code>due_date = None
  
  for item in i.item_set.all():
      if due_date is None:
          due_date = item.due_date
      else:
          if due_date &gt; item.due_date:
              due_date = item.due_date
  </code></pre>