title: Django ordering by two fields not working as expected
body: <p>i am working on a shop for clothes, shoes etc. And i am trying to sort the products by price. The problems comes when the product's discount_price field is populated which means that this product is discounted. So when i want to order the products in my view i am expecting to see products with lower discount_price before products with higher price, but it is not working like this.</p>
  
  <p>models.py</p>
  
  <pre><code>class Item(models.Model):
      price = models.IntegerField(null=True, blank=True)
      discount_price = models.IntegerField(null=True, blank=True)
  </code></pre>
  
  <p>The query i am performing</p>
  
  <pre><code>items = Item.objects.all().order_by('price', 'discount_price')
  </code></pre>