A proposal for MongoEngine-extra (or may be MongoEngine itself) to support global DB switch per thread.

XDocument

class CSDocument(object):
    _collections = defaultdict(lambda: None)
    active_db = None

    @classmethod
    def switch_active_db(cls, dbname):
        cls.active_db = dbname

    @classmethod
    def _get_db(cls):
        return get_db(cls.active_db)

    @classmethod
    def _build_key(cls, collection_name):
        thread_id = threading.currentThread().ident
        key = '%s-%s-%s' % (thread_id, cls.active_db, collection_name)
        return key

    @classmethod
    def _get_collection(cls):
        """Custom MongoEngine function to support global DB switch thread-safe.

        TODO: add support for capped collections.

        """
        collection_name = cls._get_collection_name()
        key = cls._build_key(collection_name)
        if cls._collections[key] is None:
            db = cls._get_db()
            cls._collection = db[collection_name]
            
            cls._collections[key] = db[collection_name]

            if cls._meta.get('auto_create_index', True):
                cls.ensure_indexes()


        return cls._collections[key]