Django exclude from annotation count

Question

I have following application:

from django.db import models


class Worker(models.Model):
    name = models.CharField(max_length=60)

    def __str__(self):
        return self.name


class Job(models.Model):
    worker = models.ForeignKey(Worker)
    is_completed = models.BooleanField()

I want to annotate Workers query with count of completed jobs.

I'll try to do it with following script:

from myapp.models import Worker, Job
from django.db.models import Count

w = Worker.objects.create(name='Worker1')
Job.objects.create(worker=w, is_completed=False)
Job.objects.create(worker=w, is_completed=False)
Job.objects.create(worker=w, is_completed=True)
Job.objects.create(worker=w, is_completed=True)

workers = Worker.objects.all().annotate(num_jobs=Count('job'))
workers[0].num_jobs    
# >>> 4
workers = Worker.objects.all().exclude(job__is_completed=False).annotate(num_jobs=Count('job'))
# >>> []

Result of the last query is empty. How to exclude elements from reverse relation?

Django 1.8, python 2.7

UPD. I would like to have all workers in queryset, even those, who has a zero jobs

Answer 1

update: ok I played a bit with this to generate the solution and I think I got it using Conditional Expressions:

Conditional expressions let you use if ... elif ... else logic within filters, annotations, aggregations, and updates. A conditional expression evaluates a series of conditions for each row of a table and returns the matching result expression.

Note: Conditional Expressions (such as Case and When) are new in Django 1.8, as pointed out by @Pynchia

from django.db.models import IntegerField, Sum, Case, When

workers = Worker.objects.annotate(
              num_jobs=Sum(
                  Case(
                      When(job__is_completed=True, then=1),
                      default=0,  
                      output_field=IntegerField()
                  )
               )
           )

now, each worker will have a num_jobs which will be an Integer that shows how many completed jobs that worker has :)