Unable to iterate over ManyToMany - how do I calculate the sum?

My models look like this:

class Article(models.Model):
name = models.CharField(max_length=50)
description = models.TextField()
price = models.FloatField()

def __unicode__(self):
    return "%s - %s" % (sel开发者_JS百科f.name, self.price)

class Order(models.Model):
client = models.ForeignKey(User)
articles = models.ManyToManyField(Article)
total = models.FloatField(editable=False, blank=True, default=0)

def __unicode__(self):
    return "%s - %s" % (self.client.username, self.total)

def save(self):
    for a in self.articles:
        self.total += a.price
    super(Order, self).save()

And I try to calculate the total price in save(). If I try to save an order via the admin, I get

'Order' instance needs to have a primary key value before a many-to-many relationship can be used

If I call

super(Order, self).save()

before calculating the sum, I get

'ManyRelatedManager' object is not iterable .

I don't know to do... I thought about calculating total in the view, but I would like to store it in my database.

How to I iterate over all the articles in the Order-Model?

Thanks in advance :)

---

I'd replace your total field with a property:

class Order():
    ....
    @property
    def total(self):
        return self.articles.aggregate(Sum('price'))['price__sum']

the reason is: when you add items to collection your object is not saved again, so it wouldn't update your counter. With this property you'll be sure to always have correct data.

---

If you really need to do this, you could save the instance first if it has no PK:

def save(self, *args, **kwargs):
    if self.pk is None:
        super(Order, self).save(*args, **kwargs)
    for a in self.articles.all():
        self.total += a.price
    super(Order, self).save(*args, **kwargs)

---

Use aggregation. Replace

for a in self.articles:
    self.total += a.price

with

from django.db.models import Sum

#..............
self.total = self.articles.aggregate(Sum('price'))['price__sum']