Added sorting on calculated fields chapter

docs/optimize_queries.rst

@@ -4,7 +4,7 @@ How to optimize queries in Django admin?
 If you have a lot of calculated fields in your admin, you can be running multipel queries per object leading to your admin can becoming quite slow.
 To fix this you can override the :code:`get_queryset` method on model admin to annotate the calculated fields.
 
-Lets take the example of this `ModelAdmin` we have for `Origin`::
+Lets take the example of this :code:`ModelAdmin` we have for :code:`Origin`::
 
     @admin.register(Origin)
     class OriginAdmin(admin.ModelAdmin):

docs/sorting_calculated_fields.rst

@@ -1,2 +1,42 @@
 How to enable sorting on calculated fields?
 ===========================================================
+
+Django adds sorting capabilities on fields which are attributes on the models.
+When you add a calculated field Django doesn't know how to do a :code:`order_by`, so it doesn't a sorting capability on that field.
+
+If you want to add sorting on a caluclated field, you have to tell Django, what to pass to :code:`order_by`. You can do this by setting
+:code:`admin_order_field` attribute on the calculated field method.
+
+You start from the admin you wrote in the pervious chapter (:doc:`optimize_queries`).::
+
+    hero_count.admin_order_field = '_hero_count'
+    villain_count.admin_order_field = '_villain_count'
+
+
+With these changes your admin becomes::
+
+    @admin.register(Origin)
+    class OriginAdmin(admin.ModelAdmin):
+        list_display = ("name", "hero_count", "villain_count")
+
+        def get_queryset(self, request):
+            queryset = super().get_queryset(request)
+            queryset = queryset.annotate(
+                _hero_count=Count("hero", distinct=True),
+                _villain_count=Count("villain", distinct=True),
+            )
+            return queryset
+
+        def hero_count(self, obj):
+            return obj._hero_count
+
+        def villain_count(self, obj):
+            return obj._villain_count
+
+        hero_count.admin_order_field = '_hero_count'
+        villain_count.admin_order_field = '_villain_count'
+
+
+Here is the admin sorted on :code:`hero_count`
+
+.. image:: sorting_calculated_field.png

heroes_and_monsters/entities/admin.py

@@ -33,3 +33,6 @@ def hero_count(self, obj):
 
     def villain_count(self, obj):
         return obj._villain_count
+
+    hero_count.admin_order_field = '_hero_count'
+    villain_count.admin_order_field = '_villain_count'