MongoEngine/mongoengine — pull request #277 by samuelclay (2013-04-15 21:54:28+00:00)
Title: Adding read_preference method to QuerySet

So you can make a query like this:

```python
stories = MStory.objects.filter(
story_feed_id=story_feed_id,
story_hash__exists=False
).only(
'id', 'story_feed_id', 'story_guid'
).read_preference(pymongo.ReadPreference.SECONDARY)
```