Django: Duplicated logic between properties and queryset annotations

Question

When I want to define my business logic, I\'m struggling finding the right way to do this, because I often both need a property AND a custom queryset to get the same info. I

Answer 1

I don't think there is a silver bullet here. But I use this pattern in my projects for such cases.

class PickupTimeSlotAnnotatedManager(models.Manager):
    def with_nb_bookings(self):
        return self.annotate(
            _nb_bookings=Count(
                'order', filter=Q(order__status=Order.VALIDATED)
            )
        )

class PickupTimeSlot(models.Model):
    ...
    annotated = PickupTimeSlotAnnotatedManager()

    @property
    def nb_bookings(self) -> int:
        """ How many times this time slot is booked? """ 
        if hasattr(self, '_nb_bookings'):
            return self._nb_bookings
        return self.order_set.validated().count()

In code

qs = PickupTimeSlot.annotated.with_nb_bookings()
for item in qs:
    print(item.nb_bookings)

This way I can always use property, if it is part of annotated queryset it will use annotated value if not it will calculate it. This approach guaranties that I will have full control of when to make queryset "heavier" by annotating it with required values. If I don't need this I just use regular PickupTimeSlot.objects. ...

Also if there are many such properties you could write decorator that will wrap property and simplify code. It will work as cached_property decorator, but instead it will use annotated value if it is present.