Question

I have the following django model (mapped to table ‘A’):

class A(models.Model):
    name = models.CharField(max_length=64, null=False)
    value = models.IntegerField()
    ...

I want to perform the following simple query on top:

select avg(case 
        when (value > 0 and value <= 50) then 0 
        when (value > 50 and value < 70) then 50 
        else 100 end) 
from A
where ...

I’m trying to avoid raw SQL – How can this be implemented with django (in the above example I’m using avg, but the same question is also relevant for max, min, sum etc.)?

I tried using extra and aggregate:

extra(select={'avg_field': case_when_query})

and

aggregate(Avg('avg_field')), 

but the aggregate function only works with model fields so the extra field cannot be used here.
How can this be done with django?

Thanks for the help

Answer 1

What can be done that will still allow us to use django queryset is something like this:

qs = A.objects.extra(select={"avg_field": 
                     "avg(case when...)"}).filter(...).values("avg_field")

To use the result:

qs[0]["avg_field"]

And this would allow the needed functionality.