Simple Mongo ODM

odm.py

from somewhere import get_mongo_db
# assuming get_mongo_db returns db instance

class MongoException(Exception):
    pass


class MongoField(object):
    pass


class MongoMeta(type):
    def __new__(cls, name, bases, attrs):
        super_new = super(MongoMeta, cls).__new__

        # let's create empty class:
        module = attrs.pop('__module__')
        new_class = super_new(cls, name, bases, {'__module__': module})
        new_class.mongo_fields = []

        # attributes from base classes need to be processed separately
        for base in bases:
            base_fields = getattr(base, 'mongo_fields', [])
            new_class.mongo_fields += base_fields

        # process own field attributes
        for attr, value in attrs.items():
            if isinstance(value, MongoField):
                setattr(new_class, attr, None)
                new_class.mongo_fields.append(attr)
            else:
                setattr(new_class, attr, value)
        return new_class


class MongoModel(object, metaclass=MongoMeta):  # for Python 3
    """
    Basic class for Mongo models. Add fields as MongoField attributes:
    class Person(MongoModel):
        collection_name = "people"
        name = MongoField()
        birthdate = MongoField()
    """
    __metaclass__ = MongoMeta  # for Python 2
    _id = MongoField()
    collection_name = None
    mongo_collection = None

    def __init__(self, *args, **kwargs):
        if self.collection_name:
            self.mongo_collection = get_mongo_db()[self.collection_name]
        for f in self.mongo_fields:
            setattr(self, f, kwargs.get(f))

    @classmethod
    def get(cls, _id, default={}):
        if not cls.collection_name:
            raise MongoException('Collection is not defined')
        mongo_collection = get_mongo_db()[cls.collection_name]
        mongo_data = mongo_collection.find_one(ObjectId(_id)) or default
        return cls(**mongo_data)

    def delete(self):
        if not self.mongo_collection:
            raise MongoException('Collection is not defined')
        if self._id:
            self.mongo_collection.remove(self._id)
        else:
            raise MongoException("ID is not defined")

    def save(self):
        if not self.mongo_collection:
            raise MongoException('Collection is not defined')
        mongo_data = {f: getattr(self, f) for f in self.mongo_fields}
        if not self._id:
            mongo_data.pop('_id')
        elif not isinstance(self._id, ObjectId):  # needed if we use integers as _id
            mongo_data['_id'] = unicode(self._id)
        self.mongo_collection.save(mongo_data)
        self._id = mongo_data.get('_id')