[1.6.x] Fixed #16433 -- Fixed a help_text/read only field interaction that caused an admin crash.

Thanks chris at cogdon.org for the report and admackin for the patch.

Backport of af953c45cc from master

django/contrib/admin/util.py

@@ -320,10 +320,15 @@ def label_for_field(name, model, model_admin=None, return_attr=False):
 
 
 def help_text_for_field(name, model):
-    try:
-        help_text = model._meta.get_field_by_name(name)[0].help_text
-    except models.FieldDoesNotExist:
     help_text = ""
+    try:
+        field_data = model._meta.get_field_by_name(name)
+    except models.FieldDoesNotExist:
+        pass
+    else:
+        field = field_data[0]
+        if not isinstance(field, RelatedObject):
+            help_text = field.help_text
     return smart_text(help_text)
 
 

tests/admin_views/admin.py

@@ -450,6 +450,10 @@ class GadgetAdmin(admin.ModelAdmin):
         return CustomChangeList
 
 
+class ToppingAdmin(admin.ModelAdmin):
+    readonly_fields = ('pizzas',)
+
+
 class PizzaAdmin(admin.ModelAdmin):
     readonly_fields = ('toppings',)
 
@@ -755,7 +759,7 @@ site.register(Book, inlines=[ChapterInline])
 site.register(Promo)
 site.register(ChapterXtra1, ChapterXtra1Admin)
 site.register(Pizza, PizzaAdmin)
-site.register(Topping)
+site.register(Topping, ToppingAdmin)
 site.register(Album, AlbumAdmin)
 site.register(Question)
 site.register(Answer)

tests/admin_views/models.py

@@ -495,7 +495,7 @@ class Topping(models.Model):
 
 class Pizza(models.Model):
     name = models.CharField(max_length=20)
-    toppings = models.ManyToManyField('Topping')
+    toppings = models.ManyToManyField('Topping', related_name='pizzas')
 
 
 class Album(models.Model):

tests/admin_views/tests.py

@@ -49,7 +49,7 @@ from .models import (Article, BarAccount, CustomArticle, EmptyModel, FooAccount,
     OtherStory, ComplexSortedPerson, PluggableSearchPerson, Parent, Child, AdminOrderedField,
     AdminOrderedModelMethod, AdminOrderedAdminMethod, AdminOrderedCallable,
     Report, MainPrepopulated, RelatedPrepopulated, UnorderedObject,
-    Simple, UndeletableObject, Choice, ShortMessage, Telegram)
+    Simple, UndeletableObject, Choice, ShortMessage, Telegram, Pizza, Topping)
 from .admin import site, site2
 
 
@@ -3559,6 +3559,17 @@ class ReadonlyTest(TestCase):
         self.assertContains(response, '<p>No opinion</p>', html=True)
         self.assertNotContains(response, '<p>(None)</p>')
 
+    def test_readonly_backwards_ref(self):
+        """
+        Regression test for #16433 - backwards references for related objects
+        broke if the related field is read-only due to the help_text attribute
+        """
+        topping = Topping.objects.create(name='Salami')
+        pizza = Pizza.objects.create(name='Americano')
+        pizza.toppings.add(topping)
+        response = self.client.get('/test_admin/admin/admin_views/topping/add/')
+        self.assertEqual(response.status_code, 200)
+
 
 @override_settings(PASSWORD_HASHERS=('django.contrib.auth.hashers.SHA1PasswordHasher',))
 class RawIdFieldsTest(TestCase):