Prevent exception when attempting to delete a model with a protected 1-to-1 relation

Modeladmin handles notification to the user if a model instance has protected ForeignKey
relationships. However, if the protected relation is a OneToOneField it raises an exception:

  File ".../wagtail/wagtail/contrib/modeladmin/views.py", line 742, in post
    for obj in qs.all():
AttributeError: 'MyRelatedModel' object has no attribute 'all'

because qs in this case is the related instance rather than a queryset of related instances
(as is the case for a ForeignKey).

This commit handles the OneToOneField case as well.

CHANGELOG.txt

@@ -10,6 +10,7 @@ Changelog
  * Fix: Added https support for Scribd oEmbed provider (Rodrigo)
  * Fix: Changed StreamField group labels color so labels are visible (Catherine Farman)
  * Fix: Prevented images with a very wide aspect ratio from being displayed distorted in the rich text editor (Iman Syed)
+ * Fix: Prevent exception when deleting a model with a protected One-to-one relationship (Neal Todd)
 
 
 2.6.1 (xx.xx.xxxx) - IN DEVELOPMENT

docs/releases/2.7.rst

@@ -28,6 +28,7 @@ Bug fixes
  * Added https support for Scribd oEmbed provider (Rodrigo)
  * Changed StreamField group label color so labels are visible (Catherine Farman)
  * Prevented images with a very wide aspect ratio from being displayed distorted in the rich text editor (Iman Syed)
+ * Prevent exception when deleting a model with a protected One-to-one relationship (Neal Todd)
 
 
 Upgrade considerations

wagtail/contrib/modeladmin/tests/test_simple_modeladmin.py

@@ -439,6 +439,23 @@ class TestDeleteViewWithProtectedRelation(TestCase, WagtailTestUtils):
         self.assertFalse(Author.objects.filter(id=4).exists())
 
 
+    def test_post_with_1to1_dependent_object(self):
+        response = self.post(5)
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(
+            response,
+            "'Harper Lee' is currently referenced by other objects"
+        )
+        self.assertContains(
+            response,
+            "<li><b>Solo Book:</b> To Kill a Mockingbird</li>"
+        )
+
+        # Author not deleted
+        self.assertTrue(Author.objects.filter(id=5).exists())
+
+
 class TestDeleteViewModelReprPrimary(TestCase, WagtailTestUtils):
     fixtures = ['modeladmintest_test.json']
 

wagtail/contrib/modeladmin/views.py

@@ -6,11 +6,12 @@ from django.contrib.admin.options import IncorrectLookupParameters
 from django.contrib.admin.utils import (
     get_fields_from_path, label_for_field, lookup_needs_distinct, prepare_lookup_value, quote, unquote)
 from django.contrib.auth.decorators import login_required
-from django.core.exceptions import ImproperlyConfigured, PermissionDenied, SuspiciousOperation
+from django.core.exceptions import (
+    ImproperlyConfigured, ObjectDoesNotExist, PermissionDenied, SuspiciousOperation)
 from django.core.paginator import InvalidPage, Paginator
 from django.db import models
 from django.db.models.fields import FieldDoesNotExist
-from django.db.models.fields.related import ManyToManyField
+from django.db.models.fields.related import ManyToManyField, OneToOneRel
 from django.shortcuts import get_object_or_404, redirect
 from django.template.defaultfilters import filesizeformat
 from django.utils.decorators import method_decorator
@@ -738,9 +739,17 @@ class DeleteView(InstanceSpecificView):
                 obj.field, ManyToManyField))
             for rel in fields:
                 if rel.on_delete == models.PROTECT:
-                    qs = getattr(self.instance, rel.get_accessor_name())
-                    for obj in qs.all():
-                        linked_objects.append(obj)
+                    if isinstance(rel, OneToOneRel):
+                        try:
+                            obj = getattr(self.instance, rel.get_accessor_name())
+                        except ObjectDoesNotExist:
+                            pass
+                        else:
+                            linked_objects.append(obj)
+                    else:
+                        qs = getattr(self.instance, rel.get_accessor_name())
+                        for obj in qs.all():
+                            linked_objects.append(obj)
             context = self.get_context_data(
                 protected_error=True,
                 linked_objects=linked_objects

wagtail/tests/modeladmintest/fixtures/modeladmintest_test.json

@@ -31,6 +31,14 @@
         "date_of_birth": "1898-11-29"
     }
 },
+{
+    "pk": 5,
+    "model": "modeladmintest.author",
+    "fields": {
+        "name": "Harper Lee",
+        "date_of_birth": "1926-04-28"
+    }
+},
 {
     "pk": 1,
     "model": "modeladmintest.book",
@@ -63,6 +71,14 @@
         "author_id": 3
     }
 },
+{
+    "pk": 4,
+    "model": "modeladmintest.solobook",
+    "fields": {
+        "title": "To Kill a Mockingbird",
+        "author_id": 5
+    }
+},
 {
     "pk": "boom",
     "model": "modeladmintest.token",

wagtail/tests/modeladmintest/migrations/0008_solobook.py

@@ -0,0 +1,22 @@
+# Generated by Django 2.1.10 on 2019-07-17 17:46
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('modeladmintest', '0007_friend'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='SoloBook',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('title', models.CharField(max_length=255)),
+                ('author', models.OneToOneField(on_delete=django.db.models.deletion.PROTECT, to='modeladmintest.Author')),
+            ],
+        ),
+    ]

wagtail/tests/modeladmintest/models.py

@@ -40,6 +40,14 @@ class Book(models.Model, index.Indexed):
         return self.title
 
 
+class SoloBook(models.Model):
+    author = models.OneToOneField(Author, on_delete=models.PROTECT)
+    title = models.CharField(max_length=255)
+
+    def __str__(self):
+        return self.title
+
+
 class Token(models.Model):
     key = models.CharField(max_length=40, primary_key=True)