How to add additional column to Django QuerySet

Question

I have a QuerySet with Books and I would like to add a score field to every Book result.

qs = Book.objects.all()

In raw SQL I would write:

SELECT
    *,
    (
        (SELECT COUNT(*) FROM votes WHERE value=1 AND book=b.id) - 
        (SELECT COUNT(*) FROM votes WHERE value=-1 AND book=b.id)
    ) AS score
FROM
    Book b;

How can I achieve it in Django? I tried annotate(), but it seems it's not meant for this kind of stuff.

Answer 1

Raw SQL is not the only way. You can use a Value() expression (see docs here):

from django.db.models import CharField, Value

MyModel.objects.all().annotate(mycolumn=Value('xxx', output_field=CharField()))

Answer 2

If votes possible values are only 1 and -1 you can just sum them using mentioned annotate: Book.objects.annotate(score=Sum('votes__value')).

If there is more possible values you can filter annotation by adding .filter(votes__value__range=(1,1)) to the above query.

If it's more complex you would have to use extra with select.