members/admin: sort age in waiting list only by year

Previously, the age column was sorted according to the birth date. This typically
causes singleton grouping upon multi sort with age as the primary sorting key. A
typical use case is sorting by age and application date.

Now the age column is sorted only by the numerical year value, which causes
the groups to be bigger.

jdav_web/members/admin.py

@@ -23,7 +23,7 @@ from django.contrib.contenttypes.admin import GenericTabularInline
 from django.utils.html import format_html
 from django.utils.translation import gettext_lazy as _
 from django.db.models import TextField, ManyToManyField, ForeignKey, Count,\
-    Sum, Case, Q, F, When, Value, IntegerField, Subquery, OuterRef
+    Sum, Case, Q, F, When, Value, IntegerField, Subquery, OuterRef, ExpressionWrapper
 from django.forms import Textarea, RadioSelect, TypedChoiceField, CheckboxInput
 from django.shortcuts import render
 from django.core.exceptions import PermissionDenied, ValidationError
@@ -625,6 +625,11 @@ class MemberWaitingListAdmin(CommonAdminMixin, admin.ModelAdmin):
     def has_add_permission(self, request, obj=None):
         return False
 
+    def age(self, obj):
+        return obj.birth_date_delta
+    age.short_description=_('age')
+    age.admin_order_field = 'birth_date_delta'
+
     def ask_for_wait_confirmation(self, request, queryset):
         """Asks the waiting person to confirm their waiting status."""
         for waiter in queryset:
@@ -661,7 +666,15 @@ class MemberWaitingListAdmin(CommonAdminMixin, admin.ModelAdmin):
         return custom_urls + urls
 
     def get_queryset(self, request):
-        queryset = super().get_queryset(request)
+        now = timezone.now()
+        age_expr = ExpressionWrapper(
+            Case(
+                When(birth_date__month__gte=now.month, birth_date__day__gt=now.day, then=now.year - F('birth_date__year') - 1),
+                default=now.year - F('birth_date__year'),
+            ),
+            output_field=IntegerField()
+        )
+        queryset = super().get_queryset(request).annotate(birth_date_delta=age_expr)
         return queryset.prefetch_related('invitationtogroup_set')
 
     def ask_for_registration_action(self, request, queryset):