def pre_save(self, model_instance, add):
        value = getattr(model_instance, self.attname)
        if value is None:
            return None
        return self._map(
            lambda item: self.item_field.pre_save(
                _FakeModel(self.item_field, item), add),
            value)
Gets our value from the model_instance and passes its items
dataset/ETHPy150Open django-nonrel/djangotoolbox/djangotoolbox/fields.py/AbstractIterableField.pre_save