Get comments for object using one query

Question

Is it possible to get object with comments related to it? Right now django comment framework creates query for every object which has related comments and another queries for comments owners. Can I somehow avoid this? I use django 1.4 so prefetch_related is allowed.

Answer 1

You could create a function that caches the count:

from django.contrib.contenttypes.models import ContentType
from django.contrib import comments

def get_comment_count_key(model):
    content_type = ContentType.objects.get_for_model(model)
    return 'comment_count_%s_%s' % (content_type.pk, model.pk)

def get_comment_count(model):
    key = get_comment_count_key(model)
    value = cache.get(key)
    if value is None:
        value = comments.get_model().objects.filter(
            content_type = ContentType.objects.get_for_model(model),
            object_pk = model.pk,
            site__pk = settings.SITE_ID
        ).count()
        cache.set(key, value)
    return value

You could extend the Comment model and add get_comment_count there. Or put get_comment_count as a template filter. It doesn't matter.

Of course, you would also need cache invalidation when a new comment is posted:

from django.db.models import signals
from django.contrib import comments

def refresh_comment_count(sender, instance, **kwargs):
    cache.delete(get_comment_count_key(instance.content_object))
    get_comment_count(instance.content_object)
post_save.connect(refresh_comment_count, sender=comments.get_model())
post_delete.connect(refresh_comment_count, sender=comments.get_model())

You could improve this last snippet, by using cache.incr() on comment_was_posted, and cache.decr() on post_delete but that's left as an exercise for you :)