$ pip install modular-odm

from modularodm import StoredObject, fields
from modularodm.validators import MinLengthValidator, MaxLengthValidator

class User(StoredObject):
    _meta = {"optimistic": True}
    _id = fields.StringField(primary=True, index=True)
    username = fields.StringField(required=True)
    password = fields.StringField(required=True, validate=[MinLengthValidator(8)])

    def __repr__(self):
        return "<User: {0}>".format(self.username)

class Comment(StoredObject):
    _meta = {"optimistic": True}
    _id = fields.StringField(primary=True, index=True)
    text = fields.StringField(validate=MaxLengthValidator(500))
    user = fields.ForeignField("User", backref="comments")

    def __repr__(self):
        return "<Comment: {0}>".format(self.text)

from pymongo import MongoClient
from modularodm import storage

client = MongoClient()
db = client['testdb']
User.set_storage(storage.MongoStorage(db, collection="user"))
Comment.set_storage(storage.MongoStorage(db, collection="comment"))

>>> from modularodm.query.querydialect import DefaultQueryDialect as Q
>>> u = User(username="unladenswallow", password="h0lygrai1")
>>> u.save()
>>> comment = Comment(text="And now for something completely different.", user=u)
>>> comment2 = Comment(text="It's just a flesh wound.", user=u)
>>> comment.save()
True
>>> comment2.save()
True
>>> u = User.find_one(Q("username", "eq", "unladenswallow"))
>>> u.comment__comments
[<Comment: And now for something completely different.>, <Comment: It's just a flesh wound.>]
>>> c = Comment.find(Q("text", "startswith", "And now"))[0]
>>> c.text
'And now for something completely different.'

$ brew update
$ brew install mongodb