Django Count of Items in a Field

Question

models.py

class Event(models.Model):
    name = models.CharField(max_length=20, unique=True)
    distance = models.IntegerField()
    date = models.DateField()

class Category(models.Model):
    name = models.CharField(max_length=20, unique=True)
    description = models.CharField(max_length=20, unique=True)
    isnew = models.BooleanField(default=False)

class Result(models.Model):
    event = models.ForeignKey(Event)
    category = models.ForeignKey(Category)
    score = models.IntegerField()

I want to do a query to return a count of each unique Category in the Result table, for a given Event.

What I'm doing now is something like:

results = Result.objects.filter(event=myevent)
categorycountdict = {}
for r in results:
    if r.category in categorycountdict:
        categorycountdict[r.category] += 1
    else:
        categorycountdict[r.category] = 1

Is there a better way, perhaps by query instead of python.

Answer 1

You can use annotate() with values(). This approach is shown in the docs for values(). To get the count for each category name, you could do:

from django.db.models import Count

categories = Result.objects.filter(
    event=myevent,
).order_by('category').values(
    'category__name'
).annotate(count=Count('category__name'))

This will return a list of dictionaries with keys category__name and count, for example:

[{'count': 3, 'category__name': u'category1'}, {'count': 1, 'category__name': u'category2'}]

You could convert this to a single dictionary by using a dictionary comprehension:

counts_by_category = {d['category__name']: d['count'] for f in categories}