Question

Hay, i have a simple model

class Manufacturer(models.Model):
    name = models.CharField()
    car_count = models.IntegerField()

class Car(models.Model):
    maker = ForeignKey(Manufacturer)

I want to update the car_count field when a car is added to a manufacturer, I’m aware i could just count the Manufacturer.car_set() to get the value, but i want the value to be stored within that car_count field.

How would i do this?

EDIT

Would something like this work?

    def save(self):
        if self.id:
            car_count = self.car_set.count()
            self.save()

Answer 1

The best way make something happen when a model is saved it to use a signal. Django’s documentation does a good job of describing what signals are and how to use them: http://docs.djangoproject.com/en/dev/topics/signals/

I’m not sure why you need to make it a field in the model though. Databases are very good at counting rows, so you could add a model method to count the cars which would use a very fast COUNT() query.

class Manufacturer(models.Model):
    name = models.CharField()

    def car_count(self):
        return Car.objects.filter(maker=self).count()

class Car(models.Model):
    maker = ForeignKey(Manufacturer)

In light of the requirement added by your comment, you’re back to updating a field on the Manufacturer model whenever a Car is saved. I would still recommend using the count() method to ensure the car_count field is accurate. So your signal handler could look something like this:

def update_car_count(sender, **kwargs):
    instance = kwargs['instance']
    manufacturer = instance.maker
    manufacturer.car_count = Car.objects.filter(maker=self).count()
    manufacturer.save()

Then you would connect it to both the post_save and post_delete signals of the Car model.

post_save.connect(update_car_count, sender=Car)
post_delete.connect(update_car_count, sender=Car)