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

The Docs for aggregation https://docs.djangoproject.com/en/dev/topics/db/aggregation/#cheat-sheet

There probably is a way to use aggregate or annotate, but I prefer this:

departments = Department.objects.all()
for dept in departments : 
    # Get the number of reviewed products for a given range and department
    num_products = dept.product_set.filter(review__time__range=["2012-01-01", "2012-01-08"]).count()

if you absolutely need it as a function of the model:

class Department(models.Model) :
    ...
    def num_products(self, start_date, end_date) : 
        return self.product_set.filter(review__time__range=[start_date, end_date]).count()

EDIT

I think that if you were to do a raw query (something like this)

sql = """SELECT COUNT(Product.*) as num_products, Department.*
    FROM Department
    LEFT OUTER JOIN Product ON Product.department = Department.id
    LEFT OUTER JOIN Review ON Product.id = Review.product
    WHERE Review.time BETWEEN "2012-01-01" AND "2012-01-08"
    GROUP BY Department.id"""

Department.objects.raw(sql)

and then num_products will be an attribute on every Dept instance in the results.

you may need to play with the field + table names a little