[Solved]-How to implement sorting in Django Admin for calculated model properties without writing the logic twice?

Although I think your solution is very good (or even better), the another approach can be to extract admin query to the model manager:

class DeManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().annotate(
            s_d=Case(
                When(fr=True, then='s_d'),
                When(fr=False, then=F('gd') + F('na')),
                default=Value(0),
                output_field=IntegerField(),
            )
        )


class De(models.Model):
    fr = models.BooleanField("[...]")
    de = models.SmallIntegerField("[...]")
    gd = models.SmallIntegerField("[...]")
    na = models.SmallIntegerField("[...]")
    objects = DeManager()


class DeAdmin(admin.ModelAdmin):
    list_display = ("[...]", "s_d", "gd", "na", "de", "fr" )

In this case you don’t need the property because each object will have s_d attribute, although this is true only for existing objects (from the database). If you create a new object in Python and try to access obj.s_d you will get an error. Another disadvantage is that each query will be annotated with this attribute even if you don’t use it, but this can be solved by customizing the manager’s queryset.

Unfortunately, this is impossible in current stable Django version (up to 2.2) due to Django admin not fetching admin_order_field from object properties.

Fortunately, it will be possible in upcoming Django version (3.0 and up) which should be released on 2nd of December.

The way to achieve it:

class De(models.Model):

    fr = models.BooleanField("[...]")
    de = models.SmallIntegerField("[...]")
    gd = models.SmallIntegerField("[...]")
    na = models.SmallIntegerField("[...]")
    # [several_attributes, Meta, __str__() removed for readability]

    def s_d(self):
        if self.fr:
            return self.de
        else:
            return self.gd + self.na
    s_d.admin_order_field = '_s_d'
    s_d = property(s_d)

Alternatively, you can create some decorator that will add any attribute to function, before converting it to property:

def decorate(**kwargs):
    def wrap(function):
        for name, value in kwargs.iteritems():
            setattr(function, name, value)

        return function
    return wrap

class De(models.Model):

    fr = models.BooleanField("[...]")
    de = models.SmallIntegerField("[...]")
    gd = models.SmallIntegerField("[...]")
    na = models.SmallIntegerField("[...]")
    # [several_attributes, Meta, __str__() removed for readability]

    @property
    @decorate(admin_order_field='_s_d')
    def s_d(self):
        if self.fr:
            return self.de
        else:
            return self.gd + self.na

Another possible solution might be to convert the s_d property to a model field and override the model save method to keep it up to date.

# models.py

class De(models.Model):

    fr = models.BooleanField("[...]")
    de = models.SmallIntegerField("[...]")
    gd = models.SmallIntegerField("[...]")
    na = models.SmallIntegerField("[...]")
    s_d = models.SmallIntegerField("[...]", blank=True)

    # [several_attributes, Meta, __str__() removed for readability]

    def save(self, *args, **kwargs):
        if self.fr:
            self.s_d = self.de
        else:
            self.s_d = self.gd + self.na
        super().save(*args, **kwargs)

# admin.py

class DeAdmin(admin.ModelAdmin):
    list_display = ("[...]", "s_d", "gd", "na", "de", "fr" )

The default sorting in admin.py will be applied and the value of s_d will be updated every time the model is saved.

There is a caveat to this method if you plan to do a lot of bulk operations, such as bulk_create, update, or delete.

Overridden model methods are not called on bulk operations

Note that the delete() method for an object is not necessarily called
when deleting objects in bulk using a QuerySet or as a result of a
cascading delete. To ensure customized delete logic gets executed, you
can use pre_delete and/or post_delete signals.

Unfortunately, there isn’t a workaround when creating or updating
objects in bulk, since none of save(), pre_save, and post_save are
called.

The solutions presented in answers to this question work only if the property inherits ordering from one or more db columns. In this answer I share a solution that works with any property and operates on queryset results with some limitations on multiple ordering.

I am using Django 4.1.

Given the model in the question I put a random value generation in the property just as an example of values retrieved independent from db columns so that we cannot retrieve ordering from db.

import random

class De(models.Model):

    fr = models.BooleanField("[...]")
    de = models.SmallIntegerField("[...]")
    gd = models.SmallIntegerField("[...]")
    na = models.SmallIntegerField("[...]")
    # [several_attributes, Meta, __str__() removed for readability]

    def s_d(self):
        """
        Just as example: adds a perturbation so it is not possible
        to inherit ordering by db columns
        """
        X = random.randrange(128)
        if self.fr:
            return self.de*X
        else:
            return self.gd*X + self.na
    s_d.admin_order_field = 'cielcio_to_be_removed'
    s_d = property(s_d)

The solution I have used sets ChangeList.result_list overloading the ModelAdmin.paginator. This choice has the benefit to operate on computed results while the cons of not using the database ordering.

I use also a class DeChangeList inherited from ChangeList to remove the fake field set as value of admin_order_field so that I got the admin table header clickable.

from django.contrib import admin
from django.contrib.admin.views.main import ChangeList

class DeChangeList(ChangeList):

    def get_ordering(self, request, queryset):
        """
        Removes the fake field used to show upper
        and lower arrow in changelist table header
        """
        ordering = super().get_ordering(request, queryset)
        if 'cielcio_to_be_removed' in ordering:
            ordering.remove('cielcio_to_be_removed')
        if '-cielcio_to_be_removed' in ordering:
            ordering.remove('-cielcio_to_be_removed')
        return ordering


class DeAdmin(admin.ModelAdmin):

    list_display = ("[...]", "s_d", "gd", "na", "de", "fr" )

    def get_changelist(self, request, **kwargs):
        return DeChangeList

    def get_paginator(self, request, queryset, per_page, orphans=0, allow_empty_first_page=True):
        """
        Intercepts queryset and order by values with 'sorted'
        """

        index_s_p = self.list_display.index('s_p')
        ordering = request.GET.get('o', "99999")

        instances = []
        for nf in ordering.split("."):
            reverse = int(nf) < 0
            if abs(int(nf)) == index_s_p+1:
                instances = sorted(
                    queryset,
                    key=lambda a: (a.s_p is not None if reverse else a.s_p is None, a.s_p),
                    reverse=reverse
                )

        return super().get_paginator(
                request, instances or queryset,
                per_page, orphans, allow_empty_first_page)