title: Django "NULLS LAST" for creating Indexes
body: <p>Django 1.11 and later allow using <code>F</code>-expressions for adding <code>nulls last</code> option to queries:</p>
  
  <pre><code>queryset = Person.objects.all().order_by(F('wealth').desc(nulls_last=True))
  </code></pre>
  
  <p>However, we want to use this functionality for creating Indexes. A standard Django Index inside a model definition:</p>
  
  <pre><code>indexes = [
          models.Index(fields=['-wealth']),
  ]
  </code></pre>
  
  <hr>
  
  <p>I tried something along the lines of:</p>
  
  <pre><code>indexes = [
          models.Index(fields=[models.F('wealth').desc(nulls_last=True)]),
  ]
  </code></pre>
  
  <p>which returns <code>AttributeError: 'OrderBy' object has no attribute 'startswith'</code>.</p>
  
  <p>Is this possible to do using <code>F</code>-expressions in Django?</p>