Are you just trying to get the latest session? If so, rename and use [latest](https://docs.djangoproject.com/en/dev/ref/models/querysets/#latest):

```python
try:
  chunk = models.UserSessionChunk.objects.filter(user=user).select_related(depth=1).latest()
except models.UserSessionChunk.DoesNotExist:
  chunk = None
```