Django: how to pass **kwarg to annotate()

Question

I am new to django framework and Python. I have a requirement do aggregation on the fields picked at runtime by the users

class Department(models.Model):
  code = models.CharField(max_length=10, primary_key=True)
  size = models.IntegerField(blank=True, null=True)
  budget = models.IntegerField(blank=True, null=True)

I would like to optionally do the sum of either size,budget or both using **kwarg by doing something like the following

from django.db.models import Avg, Count, Sum
# let Fname has the name of the field to sum on, set by a user input
FName='size' 
FList={}
# problem in passing the following
FList['total_size']="Sum(' "+FName+" ')" 
data=Department.objects.annotate(**FList)

But I receive the following error

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "c:\python34\lib\site-packages\django\db\models\query.py", line 802, in annotate
    is_summary=False)
  File "c:\python34\lib\site-packages\django\db\models\sql\query.py", line 1030, in add_aggregate
    field_list = aggregate.lookup.split(LOOKUP_SEP)
AttributeError: 'str' object has no attribute 'lookup'

implying from the discussion in the list that Sum is not recognized in the key, value pair

Any help in passing the **kwarg to annotate is appreciated

Answer 1

You're passing the string "Sum(blah)", hence the error.

FList['total_size'] = Sum(FName)

But really there's no reason to use kwargs here at all:

data=Department.objects.annotate(total_size=Sum(FName))

Answer 2

Simplify..

from django.db.models import Avg, Count, Sum
# let Fname has the name of the field to sum on, set by a user input
FName='size' 
data=Department.objects.annotate(total_size=Sum(FName))

Since FName is a string, you don;t need to wrap it with quotes. If you needed tto run a different function, say avg etc...

from django.db.models import Avg, Count, Sum
# let Fname has the name of the field to sum on, set by a user input
FName='size' 
function_t = Sum # or Avg, or Count
data=Department.objects.annotate(total_size=function_t(FName))

I hope this helps you get down the path

Answer 3

For those who really wanted to pass a kwarg to annotate probably 'cause it is defined dinamically you have to do the following:

annotate_filters = {
    'sum_foo': Count('foo', distinct=True),
}
if foo:
    annotate_filters['sum_foo'] = Count('bar', distinct=True)
Foo.objects.annotate(**annotate_filters)

In this way you can manage different condition and pass them to the annotate function.

Happy coding.