Question

I defined couple models in my app:

class Project(models.Model):
    title   = models.CharField(max_length=150)
    url     = models.URLField()
    manager = models.ForeignKey(User)

class Cost(models.Model):
    project = models.ForeignKey(Project)
    cost    = models.FloatField()
    date    = models.DateField()

I want to sum costs for each project and render values view.py:

from mypm.costs.models import Project, Cost
from django.shortcuts import render_to_response
from django.db.models import Avg, Sum

def index(request):
    # ...
    return render_to_response('index.html',...

What is the best way of solving such aggregation in Django ORM?

Answer 1

Alexander’s solution will give you the right result, but with one query for each project. Use
annotate to do the whole thing in a single query.

from django.db.models import Sum

annotated_projects = Project.objects.all().annotate(cost_sum=Sum('cost__cost'))
for project in annotated_projects:
    print project.title, project.cost_sum