python - Django filter query on with property fields automatically calculated

Question

There is Django Order model with property fields automatically calucated. How to do a filter query.

class Order(models.Model):

    @property
    def expire(self):
        return self.created + datetime.timedelta(days=self.days_left())

    @property
    def days_left(self):
        return self.recurrence_period  * self._recurrence_unit_days[self.recurrence_unit]

Calculation done to get 1,3,7 datetime days from today

settings.SUBSCRIPTION_EXPIRATION_REMIND = [1, 3, 7]

days = map(lambda x: datetime.date.today() + datetime.timedelta(days=x), settings.SUBSCRIPTION_EXPIRATION_REMIND)

[datetime.date(2015, 7, 28),
 datetime.date(2015, 7, 30),
 datetime.date(2015, 8, 3)]

How to filter by ORM query

Order.objects.filter(expire__in=days)

Django is throwing error.

FieldError: Cannot resolve keyword 'expire' into field.

Answer 1

No, you can't perform lookup based on model methods or properties. Django ORM does not allow that.

Queries are compiled to SQL to be sent and processed at the database level whereas properties are Python code and the database knows nothing about them. That's the reason why the Django filter only allows us to use database fields.

Can do this:

Order.objects.filter(created=..) # valid as 'created' is a model field

Cannot do this:

Order.objects.filter(expires=..) # INVALID as 'expires' is a model property

You can instead use list comprehensions to get the desired result.

[obj for obj in Order.objects.all() if obj.expire in days]

The above will give me the list of Order objects having expire value in the days list.