Join two queries in Django ORM

问题

I have a Person model which has a birthday. I would like to create a query that returns all the persons information along with an additional field that tells how many people are sharing each person's birthday. In SQL I would write it like this:

SELECT p.name, b.count FROM persons as p INNER JOIN (SELECT birthday as date, COUNT(*) AS count FROM persons GROUP_BY birthday) AS b WHERE p.birthday = b.date

With Django querysets I can do the inner select but I don't know how to do the inner join.

回答1:

Seems tough to do with the ORM (though maybe possible with extra).

You could create a dict of counts by date (max 366 values, if ignoring year):

 from django.db.models import Count
 birthdate = lambda d: d.strftime("%m-%d")

 # this runs the subquery in your SQL:
 birthdays = Person.objects.values('birthday')
 counts = birthdays.annotate(count=Count('birthday'))

 counts_by_date = {
     birthdate(r['birthday']): r['count'] 
     for r in counts
 }

 for person in Person.objects.all():
     count = counts_by_date[birthdate(person.birthday)]
     print "%d people share your birthday!" % count

回答2:

I would add a model method to get the count. You can do it using the ORM like this:

from django.db import models

class Person(models.Model):
    birthdate = models.DateField()

    def shared_count(self):
        return Person.objects.filter(birthdate=self.birthdate).exclude(pk=self.id).count()

Then you can just access the count on the Person instance like this:

my_person = Person.objects.get(pk=12)
count = my_person.shared_count()

Or access it in a template like this:

{{ my_person.shared_count }}

来源：https://stackoverflow.com/questions/31798858/join-two-queries-in-django-orm