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Inspired by Mongokit, Simplemongo shares the same concept on designing the object oriented interface: using a predefined dict to restrict structure and value type of the document. ( mongoengine represents another genre, which use a rdbms way like django orm and that make simple things complicated ) But the validation mechanism of Simplemongo are formulated to be more reasonable and explicit. Following the philosophy of how MongoDB was made to be, it provides the most scalability under the premise of simplicity, let you think that you are still using pymongo and MongoDB, not some restrained orm with many rules you must follow.

The document is currently on development, feel free to check the code or test cases if you want to learn more.


import pymongo
from simplemongo import Document

# Simplemongo won't create the connection or choose the database for you,
# you must explicitly get you database object yourself
db = pymongo.MongoClient()['mydatabase']

class User(Document):
    # Define the collection of User document class ('col' abbr 'collection')
    col = db['user']

    # Enable validation on writing the document
    __validate__ = True

    # Define the struct of the document
    struct = {
        'name': str,
        'age': int,
        'attributes': {
            'vitality': float,
            'armor': int,
            'fortune': int,

user = User({
    'name': 'reorx',
    'age': 21,
    'attributes': {
        'vitality': 100.0,
        'armor': 20,
        'fortune': 15

# The document will be validated according to ``struct`` before writing to database
# An `_id` field will be added if not exist
print user['_id']

# `user.identifier` returns {'_id': user['_id']} as the identifier of the document
# The arguments of Document's `find` method is just the same with `pymongo.Collection.find`
cursor = User.find(user.identifier)

# `cursor` support all the ways `pymongo.Cursor` instance can be operated,
# instead of dict, it returns the instance of `User` class

# `one` process a find query and return the only result of the query,
# if no result, it returns `None`, if get multiple results, it raise a `MultipleObjectsReturned` exception
fetched_user =
print fetched_user['_id'] == user['_id']

# The document data of user object can and only be changed by d[key] operation,
# dot notation ( is not supported, dict should act as dict does
user['name'] = 'Reorx'

# `update` is just the dict update, it won't hit the database

# `update_self` calls the `update` method of collection object,
# equals to: user.col.update(user.identifier, {'$set': {'attributes.armor': 30}})
user.update_self({'$set': {'attributes.armor': 30}})

# `update_changes` compares raw data with the changes we have made,
# then do a `update` operation on the original collection object,
# this line equals to:
# >>> user.col.update(user.identifier, {'$set': {'name': 'Reorx'}, '$inc': {'age': 1}})

# `remove` will remove a saved or fetched document from database,
# if the document is not written in database, an AssertErrro will be raised

A detailed example

class UserDict(StructuredDict):
    struct = {
        'name': str,
        'age': int,
        'attributes': {
            'vitality': float,
            'armor': int,
            'fortune': int,
        'slots': [str],
        'skills': [
                'name': str,
                'level': int,
                'damage': float,
                'is_primary': bool,
                'parents': [
                        'name': str,
                        'distance': int,

    required_fields = [
        'name', 'attributes.vitality', 'attributes.armor',
        'skills', '', 'skills.damage'

    strict_fields = ['slots', 'skills.damage', 'skills.level']


The validation mechanism is based on three class attributes: struct, required_fields and strict_fields

  • struct is considered the field-type checker, it only checks the type of fields in the document, ignore whether the structure of the document is matched.
  • A field defined in struct will only be checked when it exists in the document, if not exists, struct check won't be triggered.
  • A field defined in struct is allowed to be of None value.
  • A field not defined in struct will not be checked or handled, whatever value it is.

For fields defined in struct there are two extra attributes to configure validation conditions:

  1. required_fields

    A field in required_fields is required to exist in the document, if not, a KeyError exception will be raised on validation.

  2. strict_fields

    Whe a field in strict_fields exist in the docuement, its value must be strictly of the type defined in struct, that means, it could not be None unless the type is defined to be None

So there are 4 situations for a field (defined in struct firstly):

  1. not required and not strict (marked nr_ns in test code)

    it can be:

    • not exist
    • exist and value is instance of type
    • exist and value is None
  2. required and not strict (marked r_ns in test code)

    it can be:

    • exist and value is instance of type
    • exist and value is None
  3. not required and strict (marked nr_s in test code)

    it can be:

    • not exist
    • exist and value is instance of type
  4. required and strict (marked r_s in test code)

    it can only be:

    • exist and value is instance of type


A Simple & Clear MongoDB ORM




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