Following a relationship backwards, and annotating the results

Say I've got a couple of models, one related to the other via foreign key like so:

class Tourist(models.Model):
    age = models.IntegerField()

class Country(models.Model):
    tourist = models.ForeignKey(Tourist)

I know that I can work backwards from Tourist to find out how many countries he's been in using:

t = Tourist.objects.all()[0]
t.country_set.count()

But is there any way to annotate the count of countries that a tourist has visited? It would be cool if I could do something like

Tourist.objects.annotate(countries_visited=country_set.count())

I know that the last bit of "code" there makes no sense, but I'm counting on it to convey the desires of my heart. I know I c开发者_运维知识库ould just do something like this:

[(t.pk, t.country_set.count()) for t in Tourist.objects.all()]

But I'm just wondering if there is a better way to do it. Thanks!

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You are pretty close already.

from django.db.models import Count

Tourist.objects.annotate(countries_visited=Count('country'))

does what you want, even though you are following the relationship backwards.