Django: how to annotate queryset with count of filtered ForeignKey field?

Question

Django novice question :)

I have the following models - each review is for a product, and each product has a department:

class Department(models.Mod         


        

Answer 1

I've had to do a couple of similar queries in the last few days and the easiest way it to use the extra queryset function to annotate each object in your queryset with a filtered count of the products:

start = ..  # need to be formatted correctly
end = ...
departments = Departments.objects.all().extra(select = {
     'product_count' : """ SELECT COUNT(*) FROM appname_department
                           JOIN appname_product
                               ON appname_product.dept_id = appname_department.id
                           JOIN appname_review 
                               ON appname_review.product_id = appname_product.id
                           WHERE appname_review.time BETWEEN %s AND %s
                       """
}, params=[start, end])

and

{% for department in departments %}
    {{ department.product_count }}
{% endfor %}