Fixed #29981 -- Fixed inline formsets with a OnetoOneField primary key that uses to_field.

django/forms/models.py

@@ -958,8 +958,12 @@ class BaseInlineFormSet(BaseModelFormSet):
             kwargs = {
                 'label': getattr(form.fields.get(name), 'label', capfirst(self.fk.verbose_name))
             }
-            if self.fk.remote_field.field_name != self.fk.remote_field.model._meta.pk.name:
-                kwargs['to_field'] = self.fk.remote_field.field_name
+
+        # The InlineForeignKeyField assumes that the foreign key relation is
+        # based on the parent model's pk. If this isn't the case, set to_field
+        # to correctly resolve the initial form value.
+        if self.fk.remote_field.field_name != self.fk.remote_field.model._meta.pk.name:
+            kwargs['to_field'] = self.fk.remote_field.field_name
 
         # If we're adding a new object, ignore a parent's auto-generated key
         # as it will be regenerated on the save request.

tests/model_formsets_regress/models.py

@@ -16,6 +16,15 @@ class UserProfile(models.Model):
     about = models.TextField()
 
 
+class UserPreferences(models.Model):
+    user = models.OneToOneField(
+        User, models.CASCADE,
+        to_field='username',
+        primary_key=True,
+    )
+    favorite_number = models.IntegerField()
+
+
 class ProfileNetwork(models.Model):
     profile = models.ForeignKey(UserProfile, models.CASCADE, to_field="user")
     network = models.IntegerField()

tests/model_formsets_regress/tests.py

@@ -8,8 +8,8 @@ from django.forms.utils import ErrorDict, ErrorList
 from django.test import TestCase
 
 from .models import (
-    Host, Manager, Network, ProfileNetwork, Restaurant, User, UserProfile,
-    UserSite,
+    Host, Manager, Network, ProfileNetwork, Restaurant, User, UserPreferences,
+    UserProfile, UserSite,
 )
 
 
@@ -171,6 +171,14 @@ class InlineFormsetTests(TestCase):
         # Testing the inline model's relation
         self.assertEqual(formset[0].instance.user_id, "guido")
 
+    def test_inline_model_with_primary_to_field(self):
+        """An inline model with a OneToOneField with to_field & primary key."""
+        FormSet = inlineformset_factory(User, UserPreferences, exclude=('is_superuser',))
+        user = User.objects.create(username='guido', serial=1337)
+        UserPreferences.objects.create(user=user, favorite_number=10)
+        formset = FormSet(instance=user)
+        self.assertEqual(formset[0].fields['user'].initial, 'guido')
+
     def test_inline_model_with_to_field_to_rel(self):
         """
         #13794 --- An inline model with a to_field to a related field of a