[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/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