[1.6.x] Fixed #23754 -- Always allowed reference to the primary key in the admin

This change allows dynamically created inlines "Add related" button to work
correcly as long as their associated foreign key is pointing to the primary
key of the related model.

Thanks to amorce for the report, Julien Phalip for the initial patch,
and Collin Anderson for the review.

Backport of f9c4e14aec from master

tests/admin_views/admin.py

@@ -31,7 +31,7 @@ from .models import (Article, Chapter, Account, Media, Child, Parent, Picture,
     AdminOrderedCallable, Report, Color2, UnorderedObject, MainPrepopulated,
     RelatedPrepopulated, UndeletableObject, UserMessenger, Simple, Choice,
     ShortMessage, Telegram, ReferencedByParent, ChildOfReferer, M2MReference,
-    ReferencedByInline, InlineReference, InlineReferer, Ingredient)
+    ReferencedByInline, InlineReference, InlineReferer, Recipe, Ingredient, NotReferenced)
 
 
 def callable_year(dt_value):
@@ -756,7 +756,6 @@ site.register(UnorderedObject, UnorderedObjectAdmin)
 site.register(UndeletableObject, UndeletableObjectAdmin)
 site.register(ReferencedByParent)
 site.register(ChildOfReferer)
-site.register(M2MReference)
 site.register(ReferencedByInline)
 site.register(InlineReferer, InlineRefererAdmin)
 
@@ -789,7 +788,9 @@ site.register(Color2, CustomTemplateFilterColorAdmin)
 site.register(Simple, AttributeErrorRaisingAdmin)
 site.register(UserMessenger, MessageTestingAdmin)
 site.register(Choice, ChoiceList)
+site.register(Recipe)
 site.register(Ingredient)
+site.register(NotReferenced)
 
 # Register core models we need in our tests
 from django.contrib.auth.models import User, Group

tests/admin_views/models.py

@@ -690,11 +690,13 @@ class Choice(models.Model):
 
 # Models for #23329
 class ReferencedByParent(models.Model):
-    pass
+    name = models.CharField(max_length=20, unique=True)
 
 
 class ParentWithFK(models.Model):
-    fk = models.ForeignKey(ReferencedByParent)
+    fk = models.ForeignKey(
+        ReferencedByParent, to_field='name', related_name='hidden+',
+    )
 
 
 class ChildOfReferer(ParentWithFK):
@@ -704,13 +706,16 @@ class ChildOfReferer(ParentWithFK):
 class M2MReference(models.Model):
     ref = models.ManyToManyField('self')
 
+
 # Models for #23431
 class ReferencedByInline(models.Model):
-    pass
+    name = models.CharField(max_length=20, unique=True)
 
 
 class InlineReference(models.Model):
-    fk = models.ForeignKey(ReferencedByInline, related_name='hidden+')
+    fk = models.ForeignKey(
+        ReferencedByInline, to_field='name', related_name='hidden+',
+    )
 
 
 class InlineReferer(models.Model):
@@ -719,9 +724,14 @@ class InlineReferer(models.Model):
 
 # Models for #23604
 class Recipe(models.Model):
-    name = models.CharField(max_length=20)
+    pass
 
 
 class Ingredient(models.Model):
-    name = models.CharField(max_length=20)
-    recipes = models.ManyToManyField('Recipe', related_name='ingredients')
+    recipes = models.ManyToManyField(Recipe)
+
+
+# Model for #23839
+class NotReferenced(models.Model):
+    # Don't point any FK at this model.
+    pass

tests/admin_views/tests.py

@@ -591,26 +591,30 @@ class AdminViewBasicTest(AdminViewBasicTestCase):
             self.assertEqual(response.status_code, 400)
             self.assertEqual(len(calls), 1)
 
-        # Specifying a field referenced by another model should be allowed.
-        response = self.client.get("/test_admin/admin/admin_views/section/", {TO_FIELD_VAR: 'id'})
+        # #23839 - Primary key should always be allowed, even if the referenced model isn't registered.
+        response = self.client.get("/test_admin/admin/admin_views/notreferenced/", {TO_FIELD_VAR: 'id'})
         self.assertEqual(response.status_code, 200)
 
         # Specifying a field referenced by another model though a m2m should be allowed.
-        response = self.client.get("/test_admin/admin/admin_views/m2mreference/", {TO_FIELD_VAR: 'id'})
+        # XXX: We're not testing against a non-primary key field since the admin doesn't
+        # support it yet, ref #23862
+        response = self.client.get("/test_admin/admin/admin_views/recipe/", {TO_FIELD_VAR: 'id'})
         self.assertEqual(response.status_code, 200)
 
-        # #23604 - Specifying the pk of this model should be allowed when this model defines a m2m relationship
+        # #23604 - Specifying a field referenced through a reverse m2m relationship should be allowed.
+        # XXX: We're not testing against a non-primary key field since the admin doesn't
+        # support it yet, ref #23862
         response = self.client.get("/test_admin/admin/admin_views/ingredient/", {TO_FIELD_VAR: 'id'})
         self.assertEqual(response.status_code, 200)
 
         # #23329 - Specifying a field that is not refered by any other model directly registered
         # to this admin site but registered through inheritance should be allowed.
-        response = self.client.get("/test_admin/admin/admin_views/referencedbyparent/", {TO_FIELD_VAR: 'id'})
+        response = self.client.get("/test_admin/admin/admin_views/referencedbyparent/", {TO_FIELD_VAR: 'name'})
         self.assertEqual(response.status_code, 200)
 
         # #23431 - Specifying a field that is only refered to by a inline of a registered
         # model should be allowed.
-        response = self.client.get("/test_admin/admin/admin_views/referencedbyinline/", {TO_FIELD_VAR: 'id'})
+        response = self.client.get("/test_admin/admin/admin_views/referencedbyinline/", {TO_FIELD_VAR: 'name'})
         self.assertEqual(response.status_code, 200)
 
     def test_allowed_filtering_15103(self):