[FIXED] How can I include a conditional order_by in django?

Issue

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.

Solution

This is what ended up working:

MyModel.objects.annotate(
    sort_order_true=Case(
        When(bool_val=True, then=('date_a')),
        default=None
    ),
    sort_order_false=Case(
        When(bool_val=False, then=('date_b')),
        default=None
    )
).order_by(
    'sort_order_true',
    '-sort_order_false',
)

Answered By - Charlotte Mays