[1.6.x] Fixed LogEntry.get_admin_url() for non-existent models.

Regression introduced by [369b6fa]; refs #18169.

Backport of 1b47508ac8 from master

django/contrib/admin/models.py

@@ -4,7 +4,7 @@ from django.db import models
 from django.conf import settings
 from django.contrib.contenttypes.models import ContentType
 from django.contrib.admin.util import quote
-from django.core.urlresolvers import reverse
+from django.core.urlresolvers import reverse, NoReverseMatch
 from django.utils.translation import ugettext, ugettext_lazy as _
 from django.utils.encoding import smart_text
 from django.utils.encoding import python_2_unicode_compatible
@@ -74,5 +74,8 @@ class LogEntry(models.Model):
         """
         if self.content_type and self.object_id:
             url_name = 'admin:%s_%s_change' % (self.content_type.app_label, self.content_type.model)
-            return reverse(url_name, args=(quote(self.object_id),))
+            try:
+                return reverse(url_name, args=(quote(self.object_id),))
+            except NoReverseMatch:
+                pass
         return None

tests/admin_views/tests.py

@@ -1525,6 +1525,17 @@ class AdminViewStringPrimaryKeyTest(TestCase):
         self.assertEqual(counted_presence_before - 1,
                           counted_presence_after)
 
+    def test_logentry_get_admin_url(self):
+        "LogEntry.get_admin_url returns a URL to edit the entry's object or None for non-existent (possibly deleted) models"
+        log_entry_name = "Model with string primary key"  # capitalized in Recent Actions
+        logentry = LogEntry.objects.get(content_type__name__iexact=log_entry_name)
+        model = "modelwithstringprimarykey"
+        desired_admin_url = "/test_admin/admin/admin_views/%s/%s/" % (model, escape(iri_to_uri(urlquote(quote(self.pk)))))
+        self.assertEqual(logentry.get_admin_url(), desired_admin_url)
+
+        logentry.content_type.model = "non-existent"
+        self.assertEqual(logentry.get_admin_url(), None)
+
     def test_deleteconfirmation_link(self):
         "The link from the delete confirmation page referring back to the changeform of the object should be quoted"
         response = self.client.get('/test_admin/admin/admin_views/modelwithstringprimarykey/%s/delete/' % quote(self.pk))