[1.5.x] Fixed #17911 -- Ensure that admin readonly fields' display va…

…lues are shown in change forms when the raw value is None.

Backport of 29d59a8

django/contrib/admin/helpers.py

@@ -186,9 +186,7 @@ def contents(self):
                     if getattr(attr, "allow_tags", False):
                         result_repr = mark_safe(result_repr)
             else:
-                if value is None:
-                    result_repr = EMPTY_CHANGELIST_VALUE
-                elif isinstance(f.rel, ManyToManyRel):
+                if isinstance(f.rel, ManyToManyRel) and value is not None:
                     result_repr = ", ".join(map(six.text_type, value.all()))
                 else:
                     result_repr = display_for_field(value, f)

tests/regressiontests/admin_views/admin.py

@@ -27,7 +27,7 @@
     Album, Question, Answer, ComplexSortedPerson, PrePopulatedPostLargeSlug,
     AdminOrderedField, AdminOrderedModelMethod, AdminOrderedAdminMethod,
     AdminOrderedCallable, Report, Color2, UnorderedObject, MainPrepopulated,
-    RelatedPrepopulated, UndeletableObject, UserMessenger, Simple)
+    RelatedPrepopulated, UndeletableObject, UserMessenger, Simple, Choice)
 
 
 def callable_year(dt_value):
@@ -615,6 +615,12 @@ def message_extra_tags(self, request, selected):
         self.message_user(request, "Test tags", extra_tags="extra_tag")
 
 
+class ChoiceList(admin.ModelAdmin):
+    list_display = ['choice']
+    readonly_fields = ['choice']
+    fields = ['choice']
+
+
 site = admin.AdminSite(name="admin")
 site.register(Article, ArticleAdmin)
 site.register(CustomArticle, CustomArticleAdmin)
@@ -690,6 +696,7 @@ def message_extra_tags(self, request, selected):
 site.register(Color2, CustomTemplateFilterColorAdmin)
 site.register(Simple, AttributeErrorRaisingAdmin)
 site.register(UserMessenger, MessageTestingAdmin)
+site.register(Choice, ChoiceList)
 
 # Register core models we need in our tests
 from django.contrib.auth.models import User, Group

tests/regressiontests/admin_views/models.py

@@ -660,3 +660,7 @@ class Simple(models.Model):
     """
     Simple model with nothing on it for use in testing
     """
+
+class Choice(models.Model):
+    choice = models.CharField(max_length=1, blank=True, null=True,
+        choices=(('y','Yes'), ('n','No'), (None, 'No opinion')))

tests/regressiontests/admin_views/tests.py

@@ -46,7 +46,7 @@
     OtherStory, ComplexSortedPerson, Parent, Child, AdminOrderedField,
     AdminOrderedModelMethod, AdminOrderedAdminMethod, AdminOrderedCallable,
     Report, MainPrepopulated, RelatedPrepopulated, UnorderedObject,
-    Simple, UndeletableObject)
+    Simple, UndeletableObject, Choice)
 
 
 ERROR_MESSAGE = "Please enter the correct username and password \
@@ -3202,6 +3202,15 @@ def test_user_password_change_limited_queryset(self):
         response = self.client.get('/test_admin/admin2/auth/user/%s/password/' % su.pk)
         self.assertEqual(response.status_code, 404)
 
+    def test_change_form_renders_correct_null_choice_value(self):
+        """
+        Regression test for #17911.
+        """
+        choice = Choice.objects.create(choice=None)
+        response = self.client.get('/test_admin/admin/admin_views/choice/%s/' % choice.pk)
+        self.assertContains(response, '<p>No opinion</p>', html=True)
+        self.assertNotContains(response, '<p>(None)</p>')
+
 
 @override_settings(PASSWORD_HASHERS=('django.contrib.auth.hashers.SHA1PasswordHasher',))
 class RawIdFieldsTest(TestCase):