Is it possible to filter on a related item in Django annotations?

Question

I have the following 2 models:

class Job(models.Model):
    title = models.CharField(_('title'), max_length=50)
    description = models.TextField(_('description'))
    category = models.ForeignKey(JobCategory, related_name='jobs')
    created_date = models.DateTimeField(auto_now_add=True)

class JobCategory(models.Model):
    title = models.CharField(_('title'), max_length=50)
    slug = models.SlugField(_('slug'))

Here is where I am at with the query thus far:

def job_categories():
    categories = JobCategory.objects.annotate(num_postings=Count('jobs'))
    return {'categories': categories}

The problem is that I only want to count jobs that were created in the past 30 days. I want to return all categories however, not only those categories that have qualifying jobs.

Answer 1

I decided to approach this differently and chose not to use annotations at all. I added a manager to the Job model that returned only active (30 days or less old) jobs, and created a property on the JobCategory model that queried for the instance's job count. My templatetag simply returned all categories. Here is the relevant code.

class JobCategory(models.Model):
    title = models.CharField(_('title'), max_length=50, help_text=_("Max 50 chars. Required."))
    slug = models.SlugField(_('slug'), help_text=_("Only letters, numbers, or hyphens. Required."))

    class Meta:
        verbose_name = _('job category')
        verbose_name_plural = _('job categories')

    def __unicode__(self):
        return self.title

    def get_absolute_url(self):
        return reverse('djobs_category_jobs', args=[self.slug])

    @property
    def active_job_count(self):
        return len(Job.active.filter(category=self))

class ActiveJobManager(models.Manager):
    def get_query_set(self):
        return super(ActiveJobManager, self).get_query_set().filter(created_date__gte=datetime.datetime.now() - datetime.timedelta(days=30))

class Job(models.Model):
    title = models.CharField(_('title'), max_length=50, help_text=_("Max 50 chars. Required."))
    description = models.TextField(_('description'), help_text=_("Required."))
    category = models.ForeignKey(JobCategory, related_name='jobs')
    employment_type = models.CharField(_('employment type'), max_length=5, choices=EMPLOYMENT_TYPE_CHOICES, help_text=_("Required."))
    employment_level = models.CharField(_('employment level'), max_length=5, choices=EMPLOYMENT_LEVEL_CHOICES, help_text=_("Required."))
    employer = models.ForeignKey(Employer)
    location = models.ForeignKey(Location)
    contact = models.ForeignKey(Contact)
    allow_applications = models.BooleanField(_('allow applications'))
    created_date = models.DateTimeField(auto_now_add=True)

    objects = models.Manager()
    active = ActiveJobManager()

    class Meta:
        verbose_name = _('job')
        verbose_name_plural = _('jobs')

    def __unicode__(self):
        return '%s at %s' % (self.title, self.employer.name)

and the tag...

def job_categories():
    categories = JobCategory.objects.all()
    return {'categories': categories}

Answer 2

Just a guess... but would this work?

def job_categories():
    thritydaysago = datetime.datetime.now() - datetime.timedelta(days=30)
    categories = JobCategory.objects.filter(job__created_date__gte=thritydaysago).annotate(num_postings=Count('jobs'))
    return {'categories': categories}

See"lookups-that-span-relationships" for more details on spanning queries. Hmmm... probably need another query in there to get all categories...