Question

I’m new to Django and I’d like to understand how to use managers. I’ve read the documentation but need a little help putting it into practice.

I’ve got models as follows:

class Place(models.Model):
    id = models.IntegerField(primary_key=True) 
    name = models.CharField(max_length=300)
class PlaceRef(models.Model):
    place = models.ForeignKey(Place)
    entry = models.ForeignKey(Entry)
    value = models.FloatField()
    units = models.CharField(max_length=30)

If I had a particular Place, should I use a Manager to add up the value of all the PlaceRefs associated with it? (assuming for the sake of simplicity that all the units are the same…)

place = Place.objects.get(id=id)
value = PlaceRef.objects... # what to do here?

Answer 1

No need for a new manager here. There’s already an automatic manager that deals with the relationship between Place and PlaceRef – it’s accessed via place.placeref_set.

But to add up the values, you need to use aggregation – specifically, the aggregate method.

from django.db.models import Sum
value = place.placeref_set.aggregate(Sum('value'))

Now value is a dictionary with a single value containing the sum of all the values in the associated PlaceRefs.