Fixed #31181 -- Added links to related models for admin's readonly fields.

5 files changed, 87 insertions, 11 deletions

diff --git a/django/contrib/admin/helpers.py b/django/contrib/admin/helpers.py
index 56d7970d2a..6f0be6b148 100644
--- a/django/contrib/admin/helpers.py
+++ b/django/contrib/admin/helpers.py
@@ -3,12 +3,15 @@ import json
 from django import forms
 from django.contrib.admin.utils import (
     display_for_field, flatten_fieldsets, help_text_for_field, label_for_field,
-    lookup_field,
+    lookup_field, quote,
 )
 from django.core.exceptions import ObjectDoesNotExist
-from django.db.models import ManyToManyRel
+from django.db.models.fields.related import (
+    ForeignObjectRel, ManyToManyRel, OneToOneField,
+)
 from django.forms.utils import flatatt
 from django.template.defaultfilters import capfirst, linebreaksbr
+from django.urls import NoReverseMatch, reverse
 from django.utils.html import conditional_escape, format_html
 from django.utils.safestring import mark_safe
 from django.utils.translation import gettext, gettext_lazy as _
@@ -187,6 +190,17 @@ class AdminReadonlyField:
         label = self.field['label']
         return format_html('<label{}>{}{}</label>', flatatt(attrs), capfirst(label), self.form.label_suffix)
 
+    def get_admin_url(self, remote_field, remote_obj):
+        url_name = 'admin:%s_%s_change' % (
+            remote_field.model._meta.app_label,
+            remote_field.model._meta.model_name,
+        )
+        try:
+            url = reverse(url_name, args=[quote(remote_obj.pk)])
+            return format_html('<a href="{}">{}</a>', url, remote_obj)
+        except NoReverseMatch:
+            return str(remote_obj)
+
     def contents(self):
         from django.contrib.admin.templatetags.admin_list import _boolean_icon
         field, obj, model_admin = self.field['field'], self.form.instance, self.model_admin
@@ -212,6 +226,11 @@ class AdminReadonlyField:
             else:
                 if isinstance(f.remote_field, ManyToManyRel) and value is not None:
                     result_repr = ", ".join(map(str, value.all()))
+                elif (
+                    isinstance(f.remote_field, (ForeignObjectRel, OneToOneField)) and
+                    value is not None
+                ):
+                    result_repr = self.get_admin_url(f.remote_field, value)
                 else:
                     result_repr = display_for_field(value, f, self.empty_value_display)
                 result_repr = linebreaksbr(result_repr)
diff --git a/docs/releases/3.2.txt b/docs/releases/3.2.txt
index d82c9c719a..ab1d4dfb9a 100644
--- a/docs/releases/3.2.txt
+++ b/docs/releases/3.2.txt
@@ -72,6 +72,9 @@ Minor features
 * :attr:`.ModelAdmin.search_fields` now allows searching against quoted phrases
   with spaces.
 
+* Read-only related fields are now rendered as navigable links if target models
+  are registered in the admin.
+
 :mod:`django.contrib.admindocs`
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/tests/admin_views/admin.py b/tests/admin_views/admin.py
index 44623e483f..4a72e3070f 100644
--- a/tests/admin_views/admin.py
+++ b/tests/admin_views/admin.py
@@ -37,8 +37,8 @@ from .models import (
     Person, Persona, Picture, Pizza, Plot, PlotDetails, PlotProxy,
     PluggableSearchPerson, Podcast, Post, PrePopulatedPost,
     PrePopulatedPostLargeSlug, PrePopulatedSubPost, Promo, Question,
-    ReadablePizza, ReadOnlyPizza, Recipe, Recommendation, Recommender,
-    ReferencedByGenRel, ReferencedByInline, ReferencedByParent,
+    ReadablePizza, ReadOnlyPizza, ReadOnlyRelatedField, Recipe, Recommendation,
+    Recommender, ReferencedByGenRel, ReferencedByInline, ReferencedByParent,
     RelatedPrepopulated, RelatedWithUUIDPKModel, Report, Reservation,
     Restaurant, RowLevelChangePermissionModel, Section, ShortMessage, Simple,
     Sketch, Song, State, Story, StumpJoke, Subscriber, SuperVillain, Telegram,
@@ -539,6 +539,10 @@ class PizzaAdmin(admin.ModelAdmin):
     readonly_fields = ('toppings',)
 
 
+class ReadOnlyRelatedFieldAdmin(admin.ModelAdmin):
+    readonly_fields = ('chapter', 'language', 'user')
+
+
 class StudentAdmin(admin.ModelAdmin):
     search_fields = ('name',)
 
@@ -1061,6 +1065,7 @@ site.register(GenRelReference)
 site.register(ParentWithUUIDPK)
 site.register(RelatedPrepopulated, search_fields=['name'])
 site.register(RelatedWithUUIDPKModel)
+site.register(ReadOnlyRelatedField, ReadOnlyRelatedFieldAdmin)
 
 # We intentionally register Promo and ChapterXtra1 but not Chapter nor ChapterXtra2.
 # That way we cover all four cases:
diff --git a/tests/admin_views/models.py b/tests/admin_views/models.py
index ba818ea10e..eed3cae306 100644
--- a/tests/admin_views/models.py
+++ b/tests/admin_views/models.py
@@ -365,6 +365,9 @@ class Language(models.Model):
     english_name = models.CharField(max_length=50)
     shortlist = models.BooleanField(default=False)
 
+    def __str__(self):
+        return self.iso
+
     class Meta:
         ordering = ('iso',)
 
@@ -999,3 +1002,9 @@ class UserProxy(User):
     """Proxy a model with a different app_label."""
     class Meta:
         proxy = True
+
+
+class ReadOnlyRelatedField(models.Model):
+    chapter = models.ForeignKey(Chapter, models.CASCADE)
+    language = models.ForeignKey(Language, models.CASCADE)
+    user = models.ForeignKey(User, models.CASCADE)
diff --git a/tests/admin_views/tests.py b/tests/admin_views/tests.py
index 6842b02d3e..94646f2034 100644
--- a/tests/admin_views/tests.py
+++ b/tests/admin_views/tests.py
@@ -48,12 +48,12 @@ from .models import (
     Parent, ParentWithDependentChildren, ParentWithUUIDPK, Person, Persona,
     Picture, Pizza, Plot, PlotDetails, PluggableSearchPerson, Podcast, Post,
     PrePopulatedPost, Promo, Question, ReadablePizza, ReadOnlyPizza,
-    Recommendation, Recommender, RelatedPrepopulated, RelatedWithUUIDPKModel,
-    Report, Restaurant, RowLevelChangePermissionModel, SecretHideout, Section,
-    ShortMessage, Simple, Song, State, Story, SuperSecretHideout, SuperVillain,
-    Telegram, TitleTranslation, Topping, UnchangeableObject, UndeletableObject,
-    UnorderedObject, UserProxy, Villain, Vodcast, Whatsit, Widget, Worker,
-    WorkHour,
+    ReadOnlyRelatedField, Recommendation, Recommender, RelatedPrepopulated,
+    RelatedWithUUIDPKModel, Report, Restaurant, RowLevelChangePermissionModel,
+    SecretHideout, Section, ShortMessage, Simple, Song, State, Story,
+    SuperSecretHideout, SuperVillain, Telegram, TitleTranslation, Topping,
+    UnchangeableObject, UndeletableObject, UnorderedObject, UserProxy, Villain,
+    Vodcast, Whatsit, Widget, Worker, WorkHour,
 )
 
 ERROR_MESSAGE = "Please enter the correct username and password \
@@ -5042,6 +5042,45 @@ class ReadonlyTest(AdminFieldExtractionMixin, TestCase):
         response = self.client.get(reverse('admin:admin_views_choice_change', args=(choice.pk,)))
         self.assertContains(response, '<div class="readonly">No opinion</div>', html=True)
 
+    def test_readonly_foreignkey_links(self):
+        """
+        ForeignKey readonly fields render as links if the target model is
+        registered in admin.
+        """
+        chapter = Chapter.objects.create(
+            title='Chapter 1',
+            content='content',
+            book=Book.objects.create(name='Book 1'),
+        )
+        language = Language.objects.create(iso='_40', name='Test')
+        obj = ReadOnlyRelatedField.objects.create(
+            chapter=chapter,
+            language=language,
+            user=self.superuser,
+        )
+        response = self.client.get(
+            reverse('admin:admin_views_readonlyrelatedfield_change', args=(obj.pk,)),
+        )
+        # Related ForeignKey object registered in admin.
+        user_url = reverse('admin:auth_user_change', args=(self.superuser.pk,))
+        self.assertContains(
+            response,
+            '<div class="readonly"><a href="%s">super</a></div>' % user_url,
+            html=True,
+        )
+        # Related ForeignKey with the string primary key registered in admin.
+        language_url = reverse(
+            'admin:admin_views_language_change',
+            args=(quote(language.pk),),
+        )
+        self.assertContains(
+            response,
+            '<div class="readonly"><a href="%s">_40</a></div>' % language_url,
+            html=True,
+        )
+        # Related ForeignKey object not registered in admin.
+        self.assertContains(response, '<div class="readonly">Chapter 1</div>', html=True)
+
     def test_readonly_manytomany_backwards_ref(self):
         """
         Regression test for #16433 - backwards references for related objects
@@ -5071,7 +5110,8 @@ class ReadonlyTest(AdminFieldExtractionMixin, TestCase):
 
         response = self.client.get(reverse('admin:admin_views_plotproxy_change', args=(pl.pk,)))
         field = self.get_admin_readonly_field(response, 'plotdetails')
-        self.assertEqual(field.contents(), 'Brand New Plot')
+        pd_url = reverse('admin:admin_views_plotdetails_change', args=(pd.pk,))
+        self.assertEqual(field.contents(), '<a href="%s">Brand New Plot</a>' % pd_url)
 
         # The reverse relation also works if the OneToOneField is null.
         pd.plot = None