Save (create or update) the instance to the database

        :param instance: an instance of modeled data object

def save(self, instance):
        """Save (create or update) the instance to the database

        :param instance: an instance of modeled data object
        """
        cond = tinydb.where('original') == instance.original
        eid = self.update(instance, cond)
        if eid is None:
            return self.create(instance)
        return eid