[Django]-Django aggregate or annotate

by

23👍

✅

for a given area:

my_area = Area.objects.all()[0]
Event.objects.filter(area=my_area).count()

annotation

events = Event.objects.annotate(Count('area'))
for event in events:
    print event, event.area__count

or 

events = Event.objects.annotate(count=Count('area'))
for event in events:
    print event, event.count

See the following docs:

https://docs.djangoproject.com/en/dev/ref/models/querysets/#annotate

👤c4urself

21👍

If you just need the total number of events for a single area, you don’t need either annotate or aggregate, a simple count will do:

Event.objects.filter(area=my_area).count()

If you want the count of events for multiple areas, you need annotate in conjunction with values:

Event.objects.values('area').annotate(Count('area'))
👤Daniel Roseman

3👍

Thank you all very much. The problem I was having is documented in the last version, it is about the annotate and filter precedence. 

areas = Area.objects.filter(event__in = eventQuery).annotate(num=Count('event'))

My error was in the fact that I was doing annotate first and filter second.

👤freethrow

0👍

Given the models Event and Area as:

from django.db.models import Count

class Area(models.Model):
    area_name = models.CharField(...)
    address = models.CharField(...)


class Event(models.Model):
    event_name = models.CharField(...)
    area = models.ForeignKey(Area,...)

In order to get the total number of events in each area, you can do:

area_query = Area.objects.filter(yourfilter)
total_event_per_area = Event.objects.filter(area__in=area_query).values('area').annotate(Count('id'))
print(total_event_per_area)
<QuerySet [{'area': 2, 'id__count': 2}, {'area': 4, 'id__count': 3}]>
👤7guyo

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