How do I sort a Django QuerySet by an external value?

Question

I have a dict made up of (id, rank) pairs. I'd like to perform a Django query on the ids such that the resultant queryset is ordered by rank (descending).

Getting the queryset is easy:

rankings = {...}
result = MyModel.objects.filter(id__in=rankings.keys())

It seems like the answer should involve some sort of annotation that I can use as part of the order_by but I can't figure out how to get there.

EDIT: I neglected to mention that I need the result to be a QuerySet as this is part of a tastypie API pipeline.

Answer 1

My solution for Django > 1.10 and PostgreSQL > 9.5

from django.db.models import Func, Value, IntegerField, CharField
from django.contrib.postgres.fields import ArrayField


class ArrayPosition(Func):
    function = 'array_position'

    def __init__(self, items, *expressions, **extra):
        if isinstance(items[0], int):
            base_field = IntegerField()
        else:
            base_field = CharField(max_length=max(len(i) for i in items))
        first_arg = Value(list(items), output_field=ArrayField(base_field))
        expressions = (first_arg, ) + expressions
        super().__init__(*expressions, **extra)


pk_list = [234,12,23]
queryset = SomeModel.objects.filter(pk__in=pk_list, ...)\
    .annotate(ordering=ArrayPosition(pk_list, F('pk'), output_field=IntegerField()))\
    .order_by('ordering')

Answer 2

Something like this?

rankings = { 1 : 2, 2: 1, ... } # i.e. { 'id' : 'ranking', ... }
objects = list(MyModel.objects.filter(id__in=rankings.keys()))
objects.sort(key=lambda obj: rankings[obj.id])