refactor(SubmissionMixin): compute indices in Python

pretalx_musicrate/views.py

@@ -1,9 +1,6 @@
 from hmac import compare_digest
-from operator import itemgetter
 
 from django.contrib import messages
-from django.db.models import F, Max, Min, Window
-from django.db.models.functions import RowNumber
 from django.http import JsonResponse
 from django.shortcuts import redirect
 from django.urls import reverse
@@ -125,20 +122,25 @@ class SubmissionMixin(SingleObjectMixin):
     def get_url_kwargs(self, **kwargs):
         return self.request.resolver_match.kwargs | kwargs
 
+    @cached_property
+    def index_queryset(self):
+        return (
+            self.get_queryset()
+            .prefetch_related(None)
+            .only("pk", "code")
+            .order_by("created")
+        )
+
     @context
     @cached_property
     def prev(self):
-        prev = (
-            self.get_queryset()
-            .annotate(prev=Max("created"))
-            .filter(created__lt=self.submission.created, created=F("prev"))
-            .first()
-        )
-        if prev is None:
+        if self.index == 1:
             return None
         return reverse(
             self.request.resolver_match.view_name,
-            kwargs=self.get_url_kwargs(code=prev.code),
+            kwargs=self.get_url_kwargs(
+                code=self.index_queryset[self.index - 1 - 1].code
+            ),
         )
 
     @context
@@ -146,12 +148,10 @@ class SubmissionMixin(SingleObjectMixin):
     def index(self):
         return next(
             map(
-                itemgetter(1),
+                lambda s: s[0] + 1,
                 filter(
-                    lambda s: s[0] == self.submission.pk,
-                    self.get_queryset()
-                    .annotate(index=Window(expression=RowNumber(), order_by="created"))
-                    .values_list("pk", "index"),
+                    lambda s: s[1].pk == self.submission.pk,
+                    enumerate(self.index_queryset),
                 ),
             )
         )
@@ -159,22 +159,18 @@ class SubmissionMixin(SingleObjectMixin):
     @context
     @cached_property
     def count(self):
-        return self.get_queryset().count()
+        return self.index_queryset.count()
 
     @context
     @cached_property
     def next(self):
-        next = (
-            self.get_queryset()
-            .annotate(next=Min("created"))
-            .filter(created__gt=self.submission.created, created=F("next"))
-            .first()
-        )
-        if next is None:
+        if self.index == self.count:
             return None
         return reverse(
             self.request.resolver_match.view_name,
-            kwargs=self.get_url_kwargs(code=next.code),
+            kwargs=self.get_url_kwargs(
+                code=self.index_queryset[self.index - 1 + 1].code
+            ),
         )