Following the general rule that no good deed goes unpunished, I have a
question about complex aggregation / annotation or annotation across
foreign keys.
I have the following simplified model:
class Unit(models.Model):
unit = models.TextField()
class Process(models.Model):
process = models.TextField()
unit = models.ForeignKey(Unit)
class Exchange(models.Model):
input = models.ForeignKey(Process)
output = models.ForeignKey(Process)
amount = models.FloatField()
Is there a way for me to get a queryset for Process outputs which have
a sum of inputs greater than one kilogram?
You can do this (painfully) now by looping:
foo = []
for p in Process.objects.all():
if sum(Exchange.objects.filter(ouput=p,
input__unit__unit="kilogram").values_list('amount', flat=True)) > 1:
foo.append(p)
Sorry if this is a basic question, but I have been trying to figure it
out for a while, and without success.
-Chris
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