[Python] Django / Python - Querying calculated field

I have the below models, and want to run a calculation on a foreign key of allocation and then sum the amounts.

class Fundraising(models.Model):
@property
def amount_raised(self):
amount_raised = FundraisingDonation.objects.filter(
fundraising_event=self,
).aggregate(donationamount=Coalesce(Sum('donationamount'), 0.00))['donationamount']
return amount_raised

class FundraisingDonation(models.Model):
donationamount = models.DecimalField(max_digits=11, decimal_places=2, default=0)


class Testmodel(models.Model):
organisation = models.ForeignKey(Organisation, on_delete=models.CASCADE)
allocation = models.ForeignKey(Fundraising, on_delete=models.CASCADE, related_name='items')
percentshare = models.DecimalField(max_digits=11, decimal_places=2, default=0) #SET BY USER

@property
def amount(self):
amount = self.allocation.amount_raised * self.percentshare
return amount


The 'amount' model property above calculates the amount field for each model instance.

I'm trying now to sum up the amounts for each organisation on a new model, however the following doesn't work as I don't believe I can run a query on a calculated field?

class Totals(models.Model):
totals = total + totalB + totalC
@property
def total(self):
total = Testmodel.objects.filter(
organisation=self.organisation
).aggregate(amount=Coalesce(Sum('amount'), 0.00))['amount']


Is there any way to amend the last line for this to work, or to reperform the calculation in this line? I also tried aggregate(Sum(amount_raised * percentshare)), but this didn't seem to work either?

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