How can I include a conditional order_by in django?

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I need to sort two parts of a queryset differently based on a boolean field. - Rows that have bool_val == True should come first, and be sorted by date_a, ascending. - Rows that have bool_val == False should come second, and be sorted by date_b, descending.

The closest I've gotten is

MyModel.objects.all().annotate(
    sort_order=Case(
        When(bool_val=True, then=('date_a')), 
        default='date_b')
    ).order_by('-bool_val', 'sort_order')

But that sorts them all in the same order. If the value I needed to sort by were numerical, then I would multiply one set by -1 in my annotation, but they're date values, so that won't work.

I've also looked into creating two separate querysets and combining them, but union() isn't available until 1.11 and I have to support 1.10, and other methods of combining querysets that I've found either don't preserve order or don't result in a queryset (or both). (I'm not clear on whether union() preserves order or not, but it's moot, so I didn't dig into it much, either.) This has to be a django QuerySet object at the end.

django-1.10 sorting django python
3 Answers

This is an another approach for the same problem. I think that this is would work better in null cases(if true or false is not available), but the previous could be helpful in some use cases.

from django.db.models import F
from django.db.models.functions import Coalesce

MyModel.objects.order_by(
    Coalesce(F('date_a'), F('date_b')).desc()
)
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