django aggregate or annotate

Question

This is a very stupid thing, I know, but I just don't seem to get the handle on Django aggregate and annotate functions.

I have a very simple set of models: Events, Areas and Types. An event has foreign keys pointing to Area and Type. I would simply like to have the number of forthcoming events for any area and the same for any type, i.e. Area1 - 5 forthcoming events, Area2 - 6, or Type1 - 34 events and so on.

I would like to avoid writing custom SQL, and the q operator if possible.

Attainder

Have you read the documentation and actually tried to implement some of the examples? – Simas

Please show your code, so we can help you with a concrete problem. – Walloper

Answer 1

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

Cespitose

Answer 2

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'))

Flanigan

Answer 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.

Attainder

For those looking for this in the docs... Order of annotate and filter clauses – Almonry

Answer 4

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}]>

Handbag