Fixed #2856 -- Replaced some 404s with messages in admin.

Instead of a 404, return a redirect to admin index page with a message
indicating that the requested object does not exist. This avoids the
admin returning 404 from "Recent Actions" links for deleted objects.

django/contrib/admin/options.py

@@ -34,7 +34,7 @@ from django.forms.models import (
     modelform_factory, modelformset_factory,
 )
 from django.forms.widgets import CheckboxSelectMultiple, SelectMultiple
-from django.http import Http404, HttpResponseRedirect
+from django.http import HttpResponseRedirect
 from django.http.response import HttpResponseBase
 from django.template.response import SimpleTemplateResponse, TemplateResponse
 from django.urls import reverse
@@ -1389,6 +1389,19 @@ class ModelAdmin(BaseModelAdmin):
                 initial[k] = initial[k].split(",")
         return initial
 
+    def _get_obj_does_not_exist_redirect(self, request, opts, object_id):
+        """
+        Create a message informing the user that the object doesn't exist
+        and return a redirect to the admin index page.
+        """
+        msg = _("%(name)s with ID %(key)s doesn't exist. Perhaps it was deleted?") % {
+            'name': force_text(opts.verbose_name),
+            'key': escape(object_id),
+        }
+        self.message_user(request, msg, messages.WARNING)
+        url = reverse('admin:index', current_app=self.admin_site.name)
+        return HttpResponseRedirect(url)
+
     @csrf_protect_m
     def changeform_view(self, request, object_id=None, form_url='', extra_context=None):
         with transaction.atomic(using=router.db_for_write(self.model)):
@@ -1419,8 +1432,7 @@ class ModelAdmin(BaseModelAdmin):
                 raise PermissionDenied
 
             if obj is None:
-                raise Http404(_('%(name)s object with primary key %(key)r does not exist.') % {
-                    'name': force_text(opts.verbose_name), 'key': escape(object_id)})
+                return self._get_obj_does_not_exist_redirect(request, opts, object_id)
 
         ModelForm = self.get_form(request, obj)
         if request.method == 'POST':
@@ -1692,10 +1704,7 @@ class ModelAdmin(BaseModelAdmin):
             raise PermissionDenied
 
         if obj is None:
-            raise Http404(
-                _('%(name)s object with primary key %(key)r does not exist.') %
-                {'name': force_text(opts.verbose_name), 'key': escape(object_id)}
-            )
+            return self._get_obj_does_not_exist_redirect(request, opts, object_id)
 
         using = router.db_for_write(self.model)
 
@@ -1749,10 +1758,7 @@ class ModelAdmin(BaseModelAdmin):
         model = self.model
         obj = self.get_object(request, unquote(object_id))
         if obj is None:
-            raise Http404(_('%(name)s object with primary key %(key)r does not exist.') % {
-                'name': force_text(model._meta.verbose_name),
-                'key': escape(object_id),
-            })
+            return self._get_obj_does_not_exist_redirect(request, model._meta, object_id)
 
         if not self.has_change_permission(request, obj):
             raise PermissionDenied

tests/admin_views/tests.py

@@ -245,12 +245,16 @@ class AdminViewBasicTest(AdminViewBasicTestCase):
 
     def test_basic_edit_GET_string_PK(self):
         """
-        Ensure GET on the change_view works (returns an HTTP 404 error, see
-        #11191) when passing a string as the PK argument for a model with an
-        integer PK field.
+        GET on the change_view (when passing a string as the PK argument for a
+        model with an integer PK field) redirects to the index page with a
+        message saying the object doesn't exist.
         """
-        response = self.client.get(reverse('admin:admin_views_section_change', args=('abc',)))
-        self.assertEqual(response.status_code, 404)
+        response = self.client.get(reverse('admin:admin_views_section_change', args=('abc',)), follow=True)
+        self.assertRedirects(response, reverse('admin:index'))
+        self.assertEqual(
+            [m.message for m in response.context['messages']],
+            ["section with ID abc doesn't exist. Perhaps it was deleted?"]
+        )
 
     def test_basic_edit_GET_old_url_redirect(self):
         """
@@ -263,12 +267,15 @@ class AdminViewBasicTest(AdminViewBasicTestCase):
 
     def test_basic_inheritance_GET_string_PK(self):
         """
-        Ensure GET on the change_view works on inherited models (returns an
-        HTTP 404 error, see #19951) when passing a string as the PK argument
-        for a model with an integer PK field.
+        GET on the change_view (for inherited models) redirects to the index
+        page with a message saying the object doesn't exist.
         """
-        response = self.client.get(reverse('admin:admin_views_supervillain_change', args=('abc',)))
-        self.assertEqual(response.status_code, 404)
+        response = self.client.get(reverse('admin:admin_views_supervillain_change', args=('abc',)), follow=True)
+        self.assertRedirects(response, reverse('admin:index'))
+        self.assertEqual(
+            [m.message for m in response.context['messages']],
+            ["super villain with ID abc doesn't exist. Perhaps it was deleted?"]
+        )
 
     def test_basic_add_POST(self):
         """
@@ -1787,6 +1794,16 @@ class AdminViewPermissionsTest(TestCase):
         logged = LogEntry.objects.get(content_type=article_ct, action_flag=DELETION)
         self.assertEqual(logged.object_id, str(self.a1.pk))
 
+    def test_delete_view_nonexistent_obj(self):
+        self.client.force_login(self.deleteuser)
+        url = reverse('admin:admin_views_article_delete', args=('nonexistent',))
+        response = self.client.get(url, follow=True)
+        self.assertRedirects(response, reverse('admin:index'))
+        self.assertEqual(
+            [m.message for m in response.context['messages']],
+            ["article with ID nonexistent doesn't exist. Perhaps it was deleted?"]
+        )
+
     def test_history_view(self):
         """History view should restrict access."""
         # add user should not be able to view the list of article or change any of them
@@ -1828,8 +1845,12 @@ class AdminViewPermissionsTest(TestCase):
 
     def test_history_view_bad_url(self):
         self.client.force_login(self.changeuser)
-        response = self.client.get(reverse('admin:admin_views_article_history', args=('foo',)))
-        self.assertEqual(response.status_code, 404)
+        response = self.client.get(reverse('admin:admin_views_article_history', args=('foo',)), follow=True)
+        self.assertRedirects(response, reverse('admin:index'))
+        self.assertEqual(
+            [m.message for m in response.context['messages']],
+            ["article with ID foo doesn't exist. Perhaps it was deleted?"]
+        )
 
     def test_conditionally_show_add_section_link(self):
         """
@@ -3609,11 +3630,16 @@ class AdminCustomQuerysetTest(TestCase):
 
     def test_change_view(self):
         for i in self.pks:
-            response = self.client.get(reverse('admin:admin_views_emptymodel_change', args=(i,)))
+            url = reverse('admin:admin_views_emptymodel_change', args=(i,))
+            response = self.client.get(url, follow=True)
             if i > 1:
                 self.assertEqual(response.status_code, 200)
             else:
-                self.assertEqual(response.status_code, 404)
+                self.assertRedirects(response, reverse('admin:index'))
+                self.assertEqual(
+                    [m.message for m in response.context['messages']],
+                    ["empty model with ID 1 doesn't exist. Perhaps it was deleted?"]
+                )
 
     def test_add_model_modeladmin_defer_qs(self):
         # Test for #14529. defer() is used in ModelAdmin.get_queryset()