Regression when saving a ForeignKey with to_field attr. Cached model is lost.

[Maciej Gol]

Hey guys, during recent patching run I've encountered regression between Django 2.0.9 and 2.1.2.

Given models:

class Author(models.Model):
    username = models.CharField(primary_key=True)
    numeric_id = models.AutoField()

class Book(models.Model):
    author = models.ForeignKey(Author, on_delete=models.Cascade)
    author_numeric = models.ForeignKey(Author, on_delete=models.Cascade, to_field="numeric_id")

The following test fails, although it shouldn't.

class SomeTest(TestCase):
    def test_foreign_key_with_to_field_should_properly_persist_through_save(self):
        author = Author.objects.create(username="yada")
        book = Book(author=author, author_numeric=author)
        book.save()

        with self.assertNumQueries(0):
            book.author_numeric  # No queries below Django 2.1. One query for Django 2.1.

The problem comes from the save() method here:

                # If the relationship's pk was changed, clear the cached
                # relationship.
                if obj and obj.pk != getattr(self, field.attname):
                    field.delete_cached_value(self)

We compare obj.pk with book.author_numeric_id, but pk is a string (username) and author_numeric_id is an integer. It should look for to_field value, instead:

                # If the relationship's pk was changed, clear the cached
                # relationship.
                if obj and getattr(obj, field.target_field.attname) != getattr(self, field.attname):
                    field.delete_cached_value(self)