Use NoPkDescOrderedChangeList in comment's admin

This will make sure that we don't use extra ordering fields that django normally enforces and would make the query much slower. Also moved LargeTablePaginator to the new admin_helpers file

accounts/admin.py

@@ -25,19 +25,17 @@
 from django.contrib.auth.admin import UserAdmin
 from django.contrib.auth.forms import UserChangeForm
 from django.contrib.auth.models import User
-from django.core.paginator import Paginator
-from django.db import connection
 from django.forms import ValidationError
 from django.http import HttpResponseRedirect
 from django.shortcuts import render
 from django.urls import reverse
-from django.utils.functional import cached_property
 from django.utils.safestring import mark_safe
 from django_object_actions import DjangoObjectActions
 
 from accounts.forms import username_taken_by_other_user
 from accounts.models import Profile, UserFlag, EmailPreferenceType, OldUsername, DeletedUser, UserDeletionRequest, EmailBounce, GdprAcceptance
 from general import tasks
+from utils.admin_helpers import LargeTablePaginator
 
 web_logger = logging.getLogger("web")
 
@@ -103,24 +101,6 @@ def get_num_sounds(self, obj):
         return f'{obj.profile.num_sounds}'
 
 
-class LargeTablePaginator(Paginator):
-    """ We use the information on postgres table 'reltuples' to avoid using count(*) for performance. """
-    @cached_property
-    def count(self):
-        try:
-            if not self.object_list.query.where:
-                cursor = connection.cursor()
-                cursor.execute("SELECT reltuples FROM pg_class WHERE relname = %s",
-                    [self.object_list.query.model._meta.db_table])
-                ret = int(cursor.fetchone()[0])
-                return ret
-            else :
-                return self.object_list.count()
-        except :
-            # AttributeError if object_list has no count() method.
-            return len(self.object_list)
-
-
 class AdminUserForm(UserChangeForm):
 
     def clean_username(self):

comments/admin.py

@@ -21,6 +21,7 @@
 from django.contrib import admin
 
 from comments.models import Comment
+from utils.admin_helpers import NoPkDescOrderedChangeList
 
 
 @admin.register(Comment)
@@ -31,6 +32,7 @@ class CommentAdmin(admin.ModelAdmin):
     list_select_related = ('user', 'sound')
     list_filter = ('contains_hyperlink',)
     search_fields = ('comment', '=user__username', '=sound__id')
+    ordering = ('-created',)
 
     @admin.display(description='Comment')
     def get_comment_summary(self, obj):
@@ -42,3 +44,8 @@ def has_add_permission(self, request):
 
     def has_change_permission(self, request, obj=None):
         return False
+
+    def get_changelist(self, request):
+        # Use custom change list class to avoid ordering by '-pk' in addition to '-created'
+        # That would cause a slow query as we don't have a combined db index on both fields
+        return NoPkDescOrderedChangeList

utils/admin_helpers.py

@@ -0,0 +1,51 @@
+#
+# Freesound is (c) MUSIC TECHNOLOGY GROUP, UNIVERSITAT POMPEU FABRA
+#
+# Freesound is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as
+# published by the Free Software Foundation, either version 3 of the
+# License, or (at your option) any later version.
+#
+# Freesound is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# Authors:
+#     See AUTHORS file.
+#
+
+from functools import cached_property
+
+from django.core.paginator import Paginator
+from django.contrib.admin.views.main import ChangeList
+from django.db import connection
+
+
+class NoPkDescOrderedChangeList(ChangeList):
+    def get_ordering(self, request, queryset):
+        rv = super().get_ordering(request, queryset)
+        rv = list(rv)
+        rv.remove('-pk') if '-pk' in rv else None
+        return tuple(rv)
+
+
+class LargeTablePaginator(Paginator):
+    """ We use the information on postgres table 'reltuples' to avoid using count(*) for performance. """
+    @cached_property
+    def count(self):
+        try:
+            if not self.object_list.query.where:
+                cursor = connection.cursor()
+                cursor.execute("SELECT reltuples FROM pg_class WHERE relname = %s",
+                    [self.object_list.query.model._meta.db_table])
+                ret = int(cursor.fetchone()[0])
+                return ret
+            else :
+                return self.object_list.count()
+        except :
+            # AttributeError if object_list has no count() method.
+            return len(self.object_list)