The result of me trying to get mongoengine deletion rules to work, as the documentation is a little confusing (or even misleading).

mongoengine_example.py

from mongoengine import *

class QuerySetProperty(object):
    """A set of documents belonging to an entity from another collection

    Basically, provides the functionality a backref would provide."""
    def __init__(self, cls):
        self.cls = cls

    def __get__(self, instance, owner):
        return self.cls.objects(entity=instance)

class Post(Document):
    content = StringField()
    entity = ReferenceField('Entity', dbref=False)

    def __repr__(self):
        return "<Post: '{}' owned by {}>".format(self.content, self.entity)

    def __str__(self):
        return self.content

class Entity(Document):
    name = StringField()
    posts = QuerySetProperty(Post)

    def __repr__(self):
        return "<Entity: {}>".format(self.name)

    def __str__(self):
        return self.name

Entity.register_delete_rule(Post, 'entity', CASCADE)

db = connect('examples')

Entity.drop_collection()
Post.drop_collection()

sam = Entity(name="Sam").save()
post = Post(entity=sam, content="Hello world").save()

print sam.posts, Post.objects
# [<Post: 'Hello world' owned by Sam>], [<Post: 'Hello world' owned by Sam>]

sam.delete(safe=True)

print Post.objects
# []