Get count on filtered subqueries in template

Question

If I have a parent model and a child model, how can I then list all child objects of a parent object, and have a filtered count for each child object, when listing them?

To give an example, assume we have these models:

class Category(models.Model):
    ...

class SubCategory(models.Model):
    category = models.ForeignKey(Category)
    ....

class Tag(models.Model):
    ....

class Article(models.Model)
    sub_category = models.ForeignKey(SubCategory)
    tag = models.ForeignKey(Tag)

Then we have a DetailView for a Category, in which all SubCatoegories for that Category are listed. In this list, we want to have a count on each SubCategory entry, on the number of articles it contains that also is of a certain tag (that is, not a count on all articles of that SubCategory, which seems much more trivial to build). Something like this:

{% for sub_category in category.sub_category_set.all %}
    {{ sub_category.name }} -- {{ sub_category.articles_set.filter(tag='xyz').all|length }}

I guess this has to be done in the views.py and/or models.py in some way in the end. I just wrote it like this to clarify what I mean. So, how can I achieve something like this?

Answer 1

You're right that you'll have to do this in the view. You can use annotation to get the number of filtered articles for each subcategory:

from django.db.models import Count

def my_view(request, other, arguments):
    ...
    subcategories = category.sub_category_set.filter(tag__tagname='xyz') \ 
                            .annotate(num_articles=Count('article__id'))
    ...

Pass subcategories to your template context, and in your template you can do this:

{% for subcategory in subcategories %}
    {{ subcategory.name }} -- {{ subcategory.num_articles }}
{% endfor %}