DJango ORM double join with Sum - django

Question

I searched for a similar case on SO and Google with no luck.
SHORT EXPLANATION
I have transactions that belong to an account, and an account belongs to an account aggrupation.
I want to get a list of accounts aggrupations, with their accounts, and I want to know the total balance of each account (an account balance is calculated by adding all its transactions amount).
LONG EXPLANATION
I have the following models (I include mixins for the sake of completeness):
class UniqueNameMixin(models.Model):
class Meta:
abstract = True
name = models.CharField(verbose_name=_('name'), max_length=100, unique=True)
def __str__(self):
return self.name
class PercentageMixin(UniqueNameMixin):
class Meta:
abstract = True
_validators = [MinValueValidator(0), MaxValueValidator(100)]
current_percentage = models.DecimalField(max_digits=5,
decimal_places=2,
validators=_validators,
null=True,
blank=True)
ideal_percentage = models.DecimalField(max_digits=5,
decimal_places=2,
validators=_validators,
null=True,
blank=True)
class AccountsAggrupation(PercentageMixin):
pass
class Account(PercentageMixin):
aggrupation = models.ForeignKey(AccountsAggrupation, models.PROTECT)
class Transaction(models.Model):
date = models.DateField()
concept = models.ForeignKey(Concept, models.PROTECT, blank=True, null=True)
amount = models.DecimalField(max_digits=10, decimal_places=2)
account = models.ForeignKey(Account, models.PROTECT)
detail = models.CharField(max_length=100, blank=True, null=True)
def __str__(self):
return '{} - {} - {} - {}'.format(self.date, self.concept, self.amount, self.account)
I want to be able to do this in Django ORM:
select ca.*, ca2.*, sum(ct.amount)
from core_accountsaggrupation ca
join core_account ca2 on ca2.aggrupation_id = ca.id
join core_transaction ct on ct.account_id = ca2.id
group by ca2.name
order by ca.name;

Answer 1

It would appear that nesting navigation through sets is not possible:
Wrong: AccountsAggrupation.objects.prefetch_related('account_set__transaction_set')
(or any similar approach). The way to work with this is the way around: go from transaction to account and then to account_aggroupation.
But, as I needed to have a dict with account_aggroupation, pointing each key to its set of accounts (and the balance for each), I ended up doing this:
def get_accounts_aggrupations_data(self):
accounts_aggrupations_data = {}
accounts_balances = Account.objects.annotate(balance=Sum('transaction__amount'))
for aggrupation in self.queryset:
aggrupations_accounts = accounts_balances.filter(aggrupation__id=aggrupation.id)
aggrupation.balance = aggrupations_accounts.aggregate(Sum('balance'))['balance__sum']
accounts_aggrupations_data[aggrupation] = aggrupations_accounts
current_month = datetime.today().replace(day=1).date()
date = current_month.strftime('%B %Y')
total_balance = Transaction.objects.aggregate(Sum('amount'))['amount__sum']
return {'balances': accounts_aggrupations_data, 'date': date, 'total_balance': total_balance}
Note that since I'm iterating through the accounts_aggrupations, that query (self.queryset, which leads to AccountsAggrupation.objects.all()) is executed to the DB.
The rest of the queries I do, do not execute yet because I'm not iterating through them (until consuming the info at the template).
Also note that the dictionary accounts_aggrupations_data has an accounts_aggrupation object as key.