Get a single document from the database.

        All arguments to :meth:`find` are also valid arguments for
        :meth:`find_one`, although any `limit` argument will be
        ignored. Returns a single document, or ``None`` if no matching
        document is found.

        The :meth:`find_one` method obeys the :attr:`read_preference` of
        this :class:`Collection`.

        :Parameters:

          - `filter` (optional): a dictionary specifying
            the query to be performed OR any other type to be used as
            the value for a query for ``"_id"``.

          - `*args` (optional): any additional positional arguments
            are the same as the arguments to :meth:`find`.

          - `**kwargs` (optional): any additional keyword arguments
            are the same as the arguments to :meth:`find`.

              >>> collection.find_one(max_time_ms=100)

def find_one(self, filter=None, *args, **kwargs):
        """Get a single document from the database.

        All arguments to :meth:`find` are also valid arguments for
        :meth:`find_one`, although any `limit` argument will be
        ignored. Returns a single document, or ``None`` if no matching
        document is found.

        The :meth:`find_one` method obeys the :attr:`read_preference` of
        this :class:`Collection`.

        :Parameters:

          - `filter` (optional): a dictionary specifying
            the query to be performed OR any other type to be used as
            the value for a query for ``"_id"``.

          - `*args` (optional): any additional positional arguments
            are the same as the arguments to :meth:`find`.

          - `**kwargs` (optional): any additional keyword arguments
            are the same as the arguments to :meth:`find`.

              >>> collection.find_one(max_time_ms=100)
        """
        if (filter is not None and not
                isinstance(filter, collections.Mapping)):
            filter = {"_id": filter}

        cursor = self.find(filter, *args, **kwargs)
        for result in cursor.limit(-1):
            return result
        return None