[1.8.x] Fixed #24624 -- Replaced obsoleted rel.opts in admindocs view

Thanks Scott Sanders for the report, and Markus Holtermann and
Tim Graham for the reviews. Refs #24381.
Backport of 4e7ed8d0d from master.

django/contrib/admindocs/views.py

@@ -266,8 +266,8 @@ class ModelDetailView(BaseAdminDocsView):
         # Gather related objects
         for rel in opts.related_objects:
             verbose = _("related `%(app_label)s.%(object_name)s` objects") % {
-                'app_label': rel.opts.app_label,
+                'app_label': rel.related_model._meta.app_label,
-                'object_name': rel.opts.object_name,
+                'object_name': rel.related_model._meta.object_name,
             }
             accessor = rel.get_accessor_name()
             fields.append({

docs/releases/1.8.1.txt

@@ -31,3 +31,7 @@ Bugfixes
 
 * Fixed a crash with ``QuerySet.update()`` on foreign keys to one-to-one fields
   (:ticket:`24578`).
+
+* Fixed a regression in the model detail view of
+  :mod:`~django.contrib.admindocs` when a model has a reverse foreign key
+  relation (:ticket:`24624`).

tests/admin_docs/tests.py

@@ -280,6 +280,20 @@ class TestModelDetailView(AdminDocsTestCase):
             "all related %s objects" % (link % ("admin_docs.group", "admin_docs.Group"))
         )
 
+    def test_model_with_many_to_one(self):
+        link = '<a class="reference external" href="/admindocs/models/%s/">%s</a>'
+        response = self.client.get(
+            reverse('django-admindocs-models-detail', args=['admin_docs', 'company'])
+        )
+        self.assertContains(
+            response,
+            "number of related %s objects" % (link % ("admin_docs.person", "admin_docs.Person"))
+        )
+        self.assertContains(
+            response,
+            "all related %s objects" % (link % ("admin_docs.person", "admin_docs.Person"))
+        )
+
     def test_model_with_no_backward_relations_render_only_relevant_fields(self):
         """
         A model with ``related_name`` of `+` should not show backward relationship