How to filter objects for count annotation in Django?

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Consider simple Django models Event and Participant:

class Event(models.Model):
    title = models.CharField(max_length=100)

class Participant(models.Model):
    event = models.ForeignKey(Event, db_index=True)
    is_paid = models.BooleanField(default=False, db_index=True)

It"s easy to annotate events query with total number of participants:

events = Event.objects.all().annotate(participants=models.Count("participant"))

How to annotate with count of participants filtered by is_paid=True?

I need to query all events regardless of number of participants, e.g. I don"t need to filter by annotated result. If there are 0 participants, that"s ok, I just need 0 in annotated value.

The example from documentation doesn"t work here, because it excludes objects from query instead of annotating them with 0.

Update. Django 1.8 has new conditional expressions feature, so now we can do like this:

events = Event.objects.all().annotate(paid_participants=models.Sum(
    models.Case(
        models.When(participant__is_paid=True, then=1),
        default=0,
        output_field=models.IntegerField()
    )))

Update 2. Django 2.0 has new Conditional aggregation feature, see the accepted answer below.

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How to filter objects for count annotation in Django? __del__: Questions

import time
time.sleep(5)   # Delays for 5 seconds. You can also use a float value.

import time
while True:
    print("This prints once a minute.")
    time.sleep(60) # Delay for 1 minute (60 seconds).

from time import sleep
sleep(0.1) # Time in seconds

How to filter objects for count annotation in Django? __del__: Questions

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