Building up subqueries of derived django fields - django

Question

I have a few transformations I need to perform on my table before I aggregate.
I need to multiply transaction_type (which is either 1 or -1) by amount to yield a signed_amount. Then I need to sum all signed_amounts by primary_category (which is a foreign key to secondary category which is a foreign key of my table).
DEBIT = -1
CREDIT = 1
TRANSACTION_TYPE_CHOICES = (
(DEBIT, 'debit'),
(CREDIT, 'credit'),
)
class Transaction(models.Model):
amount = models.DecimalField(max_digits=7, decimal_places=2)
transaction_type = models.IntegerField(choices=TRANSACTION_TYPE_CHOICES)
secondary_category = models.ForeignKey(Secondary_Category)
class Primary_Category(models.Model):
name = models.CharField("Category Name", max_length=30)
category = models.ForeignKey(Primary_Category_Bucket)
class Secondary_Category(models.Model):
name = models.CharField("Category Name", max_length=30)
primary_category = models.ForeignKey(Primary_Category)
I'm stuck on the first bit though.
from django.db.models import Sum, Count, F
original_transactions = Transaction.objects.all()
original_transactions.signed_amount = F('transaction_type') * F('amount')
for transaction in original_transactions:
print transaction.signed_amount
When I try to sanity check that signed_amount is being calculated, I get an error that 'Transaction' object has no attribute 'signed_amount'. I don't want to save signed_amount to the database. I just want to generate it as derived field so I can calculate my totals.
How do I calculate this derived field and subsequently aggregate by primary_category.name?

Answer 1

User python decorator property on a method for class Transaction:
class Transaction(models.Model):
amount = models.DecimalField(max_digits=7, decimal_places=2)
transaction_type = models.IntegerField(choices=TRANSACTION_TYPE_CHOICES)
secondary_category = models.ForeignKey(Secondary_Category)
#property
def signed_amount(self):
return self.amount * self.transaction_type
Then for each Transaction object you can do transaction.signed_amount.
I'm not sure if the aggregation part could be done using queries, but if you don't have that many PrimaryCategory, then python would be good enough to achieve it.

Answer 2

Or you can do this.
all_transactions = Transaction.objects.all().order_by('secondary_category__primary_category_id')
sum = 0
if all_transactions:
primary_category_id = all_transactions[0].secondary_category.primary_category_id
for transaction in all_transactions:
if primary_category_id == transaction.secondary_category.primary_category_id:
sum += (transaction.amount * transaction_type)
else:
sum = (transaction.amount * transaction_type)
print sum