def comment_saved_handler(sender, instance, created, **kwargs):
    """
    Signal handler to populated comment count fields.
    Bound to 'comment_count' field, should probably be checking for a field annotation in the model.
    """
    if 'comment_count' in [f.name for f in instance.content_object._meta.fields]:
        instance.content_object.count = instance.content_object.comments.count()
        instance.content_object.save()
 
from django.contrib.comments.models import Comment
post_save.connect(comment_saved_handler, sender=Comment)