Relation with non-auto CharField primary key is not correctly persisted if the primary key is defined after assignment

[Charlie DeTar]

Given a model with a foreign key relation to another model that has a non-auto CharField as its primary key:

class Product(models.Model):
    sku = models.CharField(primary_key=True, max_length=50)

class Order(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE)

If the relation is initialized on the parent with an empty instance that does not yet specify its primary key, and the primary key is subsequently defined, the parent does not "see" the primary key's change:

with transaction.atomic():
    order = Order()
    order.product = Product()
    order.product.sku = "foo"
    order.product.save()
    order.save()

    assert Order.objects.filter(product_id="").exists()   # Succeeds, but shouldn't
    assert Order.objects.filter(product=order.product).exists()  # Fails

Instead of product_id being populated with product.sku, it is set to emptystring. The foreign key constraint which would enforce the existence of a product with sku="" is deferred until the transaction commits. The transaction does correctly fail on commit with a ForeignKeyViolation due to the non-existence of a product with emptystring as its primary key.

On the other hand, if the related unsaved instance is initialized with its primary key before assignment to the parent, it is persisted correctly:

with transaction.atomic():
    order = Order()
    order.product = Product(sku="foo")
    order.product.save()
    order.save()

    assert Order.objects.filter(product=order.product).exists()  # succeeds

Committing the transaction also succeeds.

This may have something to do with how the Order.product_id field is handled at assignment, together with something about handling fetching of auto vs non-auto primary keys from the related instance.