Django: newbie question on Model managers

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 simplicit开发者_JAVA技巧y that all the units are the same...)

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

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.

You can also add this as an attribute of place

class Place(models.Model):
    id = models.IntegerField(primary_key=True) 
    name = models.CharField(max_length=300)

    @property
    def placeref_total(self):
      do your calculation here as suggested in Daniel's answer
      value = self.placeref_set.aggregate(Sum('value'))
      return value

then you can refer to it in your code:

  myplace = Place.objects.get(id=x)
  myplace.placeref_total

If you don't use the @property decorator, then refer to it as: myplace.placeref_total()