6.1: save() falls back to python default for primary key that should come from related instance instead

[Jacob Walls]

When saving an object whose primary key is a OneToOneField, and when that value is meant to come from an unsaved related object, and when that related object uses db_default, a2348c85fc6c20087935c74cd99340dd4ef2dcdc has the unintentional effect of falling back to the field's default instead of getting it from the related object once saved.

To demonstrate, adjust a test being drafted in this ​other PR (where the bulk_create() case was considered and handled) to use save() instead:

Passes before a2348c85fc6c20087935c74cd99340dd4ef2dcdc on SQLite. Other databases have other behaviors, like raising integrity errors, so we might need some test skips here.

tests/bulk_create/models.py

@@ -157 +157 @@
 
     class Meta:
         required_db_features = {"supports_expression_defaults"}
+
+
+class IntegerDbDefaultPrimaryKey(models.Model):
+    id = models.IntegerField(primary_key=True, db_default=42)
+
+    class Meta:
+        required_db_features = {"supports_expression_defaults"}
+
+
+class RelatedToDbDefaultPrimaryKey(models.Model):
+    related = models.OneToOneField(
+        IntegerDbDefaultPrimaryKey,
+        on_delete=models.CASCADE,
+        primary_key=True,
+        default=1,
+    )
+
+    class Meta:
+        required_db_features = {"supports_expression_defaults"}

tests/bulk_create/tests.py

@@ -27 +27 @@
     DbDefaultModel,
     DbDefaultPrimaryKey,
     FieldsWithDbColumns,
+    IntegerDbDefaultPrimaryKey,
     NoFields,
     NullableFields,
     Pizzeria,
@@ -35 +36 @@
     ProxyMultiProxyCountry,
     ProxyProxyCountry,
     RelatedModel,
+    RelatedToDbDefaultPrimaryKey,
     Restaurant,
     SmallAutoFieldModel,
     State,
@@ -439 +441 @@
         child = NullableFields.objects.get(integer_field=88)
         self.assertEqual(child.auto_field, parent)
 
+    @skipUnlessDBFeature(
+        "can_return_columns_from_insert", "supports_expression_defaults"
+    )
+    def test_pk_from_related_instance_saved_after_init_with_defaults(self):
+        # Ensure that the related field's Python default references a
+        # different valid object.
+        IntegerDbDefaultPrimaryKey.objects.create(pk=1)
+        related_object = IntegerDbDefaultPrimaryKey()
+        obj = RelatedToDbDefaultPrimaryKey(related=related_object)
+        related_object.save()
+        obj.save()
+        self.assertEqual(obj.pk, related_object.pk)
+
     def test_unsaved_parent(self):
         parent = NoFields()
         msg = (

======================================================================
FAIL: test_pk_from_related_instance_saved_after_init_with_defaults (bulk_create.tests.BulkCreateTests.test_pk_from_related_instance_saved_after_init_with_defaults)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jwalls/django/tests/bulk_create/tests.py", line 455, in test_pk_from_related_instance_saved_after_init_with_defaults
    self.assertEqual(obj.pk, related_object.pk)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 1 != 42

----------------------------------------------------------------------
Ran 1 test in 0.007s

FAILED (failures=1)