def add_logger(self, logger):
        assert logger.group is None, 'Logger already belongs to a group'
        logger.group = self
        self.loggers.append(logger)
If the model field has 'blank=True' or 'null=True' then
dataset/ETHPy150Open getlogbook/logbook/logbook/base.py/LoggerGroup.add_logger--crossover with--dataset/ETHPy150Open madisona/django-google-maps/django_google_maps/fields.py/GeoPt.__init__