Filter with field has relationship in Django?

Question

I have these models in Django:

class Customer(models.Model):
    def __unicode__(self):
        return self.name
    name = models.CharField(max_length=200)

class Sale(models.Model):
    def __unicode__(self):
        return "Sale %s (%i)" % (self.type, self.id)
    customer = models.ForeignKey(Customer)
    total = models.DecimalField(max_digits=15, decimal_places=3)
    notes = models.TextField(blank=True, null=True)

class Unitary_Sale(models.Model):
    book = models.ForeignKey(Book)
    quantity = models.IntegerField()
    unit_price = models.DecimalField(max_digits=15, decimal_places=3)
    sale = models.ForeignKey(Sale)

How can I filter to get all books that customer sale?

I tried:

units=Unitary_Sale.objects.all()
>>> units=Unitary_Sale.objects.all()
>>> for unit in units:
...    print unit.sale.customer
...    print unit.book,unit.sale.total
...
Sok nara
Khmer Empire (H001) 38.4
Sok nara
killing field (H001) 16

San ta
khmer krom (H001) 20
San ta
Khmer Empire (H001) 20
>>>

what I want:

  sok nora:56.4 (38.4+18)
  san ta:40 (20+20)

Or to dictionary:

{sok nora:156.4, san ta:40}

>>> store_resulte = {}
>>> for unit in units:
...    store_resulte[unit.sale.customer] = unit.sale.total
...
>>> print store_resulte

    {<Customer: Sok nara>: Decimal("16"), <Customer: san ta>: Decimal("20")}

It should be:

{<Customer: Sok nara>: Decimal("56.4"), <Customer: san ta>: Decimal("40")}

Answer 1

Have a look at aggregation documentation.

I believe this should do the trick (only with version 1.1 or dev.):

Customer.objects.annotate(total=Sum('sale__total'))

EDIT: You can also define a custom method for the class:

class Customer(models.Model):
    def __unicode__(self):
        return self.name
    name = models.CharField(max_length=200)

    def total_sale(self):
        total = 0
        for s in self.sale_set:
           total += s.total
        return total