title: django annotate on select_related field
body: <p>My simplified models:</p>
  
  <pre class="lang-py prettyprint-override"><code>class Product(models.Model):
      name = models.CharField()
  
  class Price(models.Model):
      product = models.OneToOneField('Product', primary_key=True)
      value = models.DecimalField()
  
  class Cart(models.Model):
      product = models.ForeignKey('Product')
      qnt =  models.IntegerField()
  </code></pre>
  
  <p>I need multiplication of two fields get stored in other field namely <code>sum</code> . Why does  <code>Cart.objects.select_related('product__price').annotate(sum=F('product__price__value') * F('qnt'))</code> returns nothing?</p>
  
  <p>Replacing <code>F('')</code> to <code>F('value')</code> returns error</p>
  
  <blockquote>
    <p>Cannot resolve keyword 'value' into field. Choices are: cart_id,
    id, product, product_id, qnt</p>
  </blockquote>