Hi all,
I've been redirected to the mailing list from the ticked I opened [0].
Therein, I proposed to add a new method to the Q object API for prefixing Q
objects with a related field's name in order to be able to create reusable
Q objects.
I think the use case I pointed out might have been misunderstood. Such an
API would allow writing a manager like this one, let's say for a
Subscription model:
class SubscriptionManager(Manager):
def active_q(self):
return Q(cancelled=False) & Q(end_date__lt=datetime.datetime.now())
# This one is just for convenience and compatibility with the evolved
practice
def active(self):
return self.filter(self.active_q())
We could then perform the following lookups:
# All active subscriptions, nothing special here
Subscription.objects.active()
# Given that a user has a ManyToManyField subscriptions, get all users with
active subscriptions, without repeating any business logic
User.objects.filter(Subscription.objects.active_q().prefix("subscriptions"))
The traditional approach would be to implement methods for filtering the
queryset on both the managers for Subscription and User and thus
duplicating logic, or use sub-queries like so:
User.objects.filter(subscriptions__in=Subscription.objects.active())
which is clearly not wanted because of performance.
What are your opinions?
Best regards
Robert
[0] https://code.djangoproject.com/ticket/30631
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