gistfile1.py

from mongoengine import *
from mongoengine.queryset import QuerySet

class PostQuerySet(QuerySet):

    def delete(self, safe=False):
        ids = [post.id for post in self]
        Comment.objects(post__in=ids).delete(safe=safe)
        super(PostQuerySet, self).delete(safe=safe)


class Post(Document):
    title = StringField()
    meta = {'queryset_class': PostQuerySet}
    
    def delete(self, safe=False):
        Comment.objects(post=self.id).delete(safe=safe)
        super(Post, self).delete(safe=safe)


class Comment(Document):
    post = ReferenceField(Post)

I think that this might just work, but the implementation is very ad hoc. I'd have to know upfront which objects refer to the Post document, which is not really useful when building extensible applications. I've forked this Gist and I'm preparing a patch that implements the idea (at least in pseudo code for now) that solves this problem in a general sense. Hang on.