Question

I would like to be able to programmatically decide which aggregator to use in a django annotate function. That is, given a statement like the following…

partsPerMachine = Machine.objects.annotate(num_parts=Count('parts'))

… I would like to be able to determine at run-time which aggregator to use. I could be summing/counting/averaging/etc.’ing parts and I won’t know which ahead of time.

I don’t quite understand the python magic that django’s authors have used to create this syntax. If this were just a case of named parameters, I could use Python’s ** syntax to pass a dictionary of parameters. But this seems to be something else…

Answer 1

The aggregators are simply Python classes. One easy way to select the desired aggregator at runtime would be to put them in a dictionary, then select the desired aggregator by key:

from django.db.models import Count, Sum, Avg

aggregators = {
    'count': Count,
    'sum': Sum,
    'avg': Avg
}

my_aggregator = aggregators[my_key]
partsPerMachine = Machine.objects.annotate(num_parts=my_aggregator('parts'))