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

To avoid any duplication, one option could be:

• remove the property in the Model
• use a custom Manager
• override it's get_queryset() method:

class PickupTimeSlotManager(models.Manager):

    def get_queryset(self):
        return super().get_queryset().annotate(
            db_nb_bookings=Count(
                'order', filter=Q(order__status=Order.VALIDATED)
            )
        )

from django.db import models
from .managers import PickupTimeSlotManager

class PickupTimeSlot(models.Model):
    ...
    # Add custom manager
    objects = PickupTimeSlotManager()

advantage: the calculated properties is transparently added to any queryset; no further action is required to use it

disadvantage: the computational overhead occurs even when the calculated property is not used