Django annotate on BooleanField - django

Question

I have the following models:
class Foo(models.Model):
pass
class Bar(models.Model):
foo = models.ForeignKey(Foo)
is_successful = models.BooleanField()
I would like to get all foo objects with an annotation if all of the bar objects associated with foo object have is_successful as True
So far my queryset is:
foos = Foo.objects.all().annotate(all_successful=Min('bar__is_successful'))
The idea for the all_successful annotation is that if the minimum value of all is_successful rows is 1, then all of them must be True (assuming 0 is False and 1 is True). So knowing that I can use the queryset like so:
foo = foos[0]
if foo.all_successful == 1:
print 'All bars are successful'
else:
print 'Not all bars are successful'
This works great in sqlite however it fails in PostgreSQL because PostgreSQL can't execute MIN aggregate on a boolean column. I guess this works in sqlite because sqlite treats bools as integers hence it can execute the aggregate.
My question is how can I make this queryset work in PostgreSQL without converting my is_successful field to an IntegerField?
Thanx

Answer 1

I know this is an old question, but I ran up against this recently. Django v1.8 now has built in support for case/when, so you can use the ORM instead of hacking around with custom SQL.
https://docs.djangoproject.com/en/1.8/ref/models/conditional-expressions/#case
Foo.objects.annotate(
all_successful=Case(
When(bar__is_successful=False, then=False),
When(bar__is_successful=True, then=True),
default=False,
output_field=BooleanField()
))
I haven't tried this out, but something similar worked for me on a recent project.

Answer 2

FOR DJANGO <= 1.7: to get an annotation I think you can simply use Extra
foos = Foo.objects.extra(select={'all_successful': 'CASE WHEN COUNT(b.foo) > 0 THEN 0 ELSE 1 END FROM yourapp_bar as b WHERE b.is_successful = false and b.foo = yourapp_foo.id' })
if your system is running Django 1.8+ please follow Dav3xor answer.

Answer 3

being inspired by https://docs.djangoproject.com/en/dev/topics/db/managers/ I suggest to use a custom manager for Bar class instead of annotation
class BarManager(models.Manager):
def get_all_successful_foos_ids(self):
from django.db import connection
cursor = connection.cursor()
cursor.execute("""
SELECT foo, COUNT(*)
FROM yourapp_bar
GROUP BY 1
WHERE is_successful = true""") # <-- you have to write the correct table name here
result_list = []
for row in cursor.fetchall():
if row[1] > 0:
result_list.append(row[0])
return result_list
class Bar(models.Model):
foo = models.ForeignKey(Foo)
is_successful = models.BooleanField()
objects = BarManager() # here I'm changing the default manager
then, in your code:
foos = foo.objects.filter(id__in=Bar.objects.get_all_successful_foos_ids())