'Set update on the specified item, and raises ItemNotFoundError
if the location doesn\'t exist'

def _update_single_item(self, location, update, allow_not_found=False):

    bulk_record = self._get_bulk_ops_record(location.course_key)
    bulk_record.dirty = True
    result = self.collection.update({'_id': location.to_deprecated_son()}, {'$set': update}, multi=False, upsert=allow_not_found, w=1)
    if (result['n'] == 0):
        raise ItemNotFoundError(location)
