bulk_create() does not fetch primary keys if they are generated by database

[Dmitry Shachnev]

In our project, we use a function that generates UUID on database side (it’s uuid7 function from PostgreSQL 18).

The bulk_create function uses _is_pk_set() method to check if the object already has a primary key assigned. However, _is_pk_set() wrongly returns True in this case: primary key is not None, however it is a DatabaseDefault instance and does not actually return a value.

Because of this, the objects returned by bulk_create do not actually have an ID.

I prepared a patch to fix this, and wrote a test case for it. Without my patch, the test fails with the following error:

======================================================================
FAIL: test_db_default_primary_key (bulk_create.tests.BulkCreateTests.test_db_default_primary_key)
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Traceback (most recent call last):
  File "/home/dmitry/upstream/django/tests/bulk_create/tests.py", line 878, in test_db_default_primary_key
    self.assertIsInstance(obj1.id, UUID)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
AssertionError: <django.db.models.expressions.DatabaseDefault object at 0x7f2563f6b570> is not an instance of <class 'uuid.UUID'>

----------------------------------------------------------------------

[Simon Charette]

Not a release blocker as db_default was introduced in 5.0 and I could reproduce the issue against it.

[Simon Charette]

Patch is LGTM. Left a comment about a potential improvement to object partitioning by primary key that could be done in a follow up optimization commit while we're around.

[Sarah Boyce <42296566+sarahboyce@…>]

In 77b4ecb:

Fixed #36260 -- Made bulk_create() work with DB-generated primary keys.

Co-authored-by: Simon Charette <charette.s@…>

[Sarah Boyce <42296566+sarahboyce@…>]

In 7d9aab8:

Refs #36260 -- Moved _is_pk_set checks into _prepare_for_bulk_create().

To avoid looping over objs twice.

[GitHub <noreply@…>]

In 02e7a16:

Refs #36088, Refs #36260 - Added supports_expression_defaults checks in bulk_create() tests.