title: django rest patch, how do I save with updated data?
body: <p>For a REST PATCH, I want to update some values in the model that already exists and save it back out.  However, I'm obviously missing some step here:</p>
  
  <ol>
  <li>Find the item to update</li>
  <li>Update the the value(s)</li>
  <li>Save it out</li>
  </ol>
  
  <p>Seems simple enough:</p>
  
  <pre><code>def patch( self, request, serialnumber, format = None ):
          try:
              r = Rack.objects.get( serialNumber = serialnumber )
          except Rack.DoesNotExist:
              data = { 'error' : 'item not found' }
              return Response( data, status = status.HTTP_404_NOT_FOUND )
  
          s = RackSerializer( r )
  
          token = "New value for token"
  
          s.save( token = token, serialNumber = serialnumber )
  
          data = { 'serialNumber' : serialnumber, 'token' : token }
          return Response( data, status = status.HTTP_200_OK )
  </code></pre>
  
  <p>This gives <code>"AssertionError: You must call '.is_valid()' before calling '.save().'"</code></p>
  
  <p>If I add the s.is_valid() call, I get <code>"Cannot call '.is_valid()' as no 'data=' keyword argument was passed when instantiating the serializer instance."</code></p>
  
  <p>Can someone point me at how to untie this knot?</p>