title: Django filter with annotate
body: <pre><code>class Review(models.Model):
      slug = models.SlugField(max_length=255, unique=True)
      vendor = models.ForeignKey(Vendor)
      user = models.ForeignKey(User, blank=True, null=True)
      product = models.ForeignKey(Product, blank=True, null=True)
      images = models.ManyToManyField(ReviewImage, blank=True, null=True)
      headline = models.CharField(max_length=100)
      review = models.TextField(blank=True, null=True)
      rating = models.IntegerField()
      active = models.BooleanField(default=1)
      created = models.DateTimeField(auto_now_add=True)
      changed = models.DateTimeField(auto_now=True)
  
  # This is the problem... I works, but no vendor is shown if there is no review.
  vendor_list = (Vendor.objects.filter(category=category,
                                       review__product__isnull=True,
                                       active=True)
                 .annotate(rating_avg=Avg('review__rating')))
  </code></pre>
  
  <p>HOW can I do it with <code>review__product__isnull=True</code>? If there is no review at all, I still want the vendor, but the rating should be: "0", .. what to do?</p>