Wierd caching behaviour in django

Question

I am using django's local memory cache (django.core.cache.backends.locmem.LocMemCache) and using tastypie for REST API.

I have following code in CommentResource object in file api.py:

def get_object_list(self, request):
    cached_comments = cache.get('comments')
    if cached_comments is not None:
        print 'got comments in cache'
        return cached_comments
    print "didn't get comments in cache"
    comments = super(CommentResource, self).get_object_list(request).order_by('-created')
    cache.set('comments', comments, 100)
    return comments

Now when I save a new comment, the API returns the new comment too, even though it prints got comments in cache. How can the new comment possibly be cached? There must be something wrong. Could the tastypie's dehydrate method be an issue here?

UPDATE: I commented out the dehydrate method and still the same issue. So that might not be the problem.

UPDATE 2: Even though, django's querysets are lazy, this answer says that

Querysets are lazy, which means they don't call the database until they're evaluated. One way they could get evaluated would be to serialize them, which is what cache.set does behind the scenes.

So laziness of querysets doesn't explain this behaviour.