`Model.__eq__` treats distinct unsaved composite-PK instances as equal

[Denny Biasiolli]

Model.__eq__ still uses pk is None to decide whether an instance is unsaved. That is wrong for composite primary keys (pk is a tuple) and for db_default PKs (DatabaseDefault is not None).

>>> User() == User()
True
>>> User(tenant_id=1) == User(tenant_id=1)
True

Documented contract (and simple-PK behavior since #18864 / #18250) is that unsaved instances are equal only to themselves. __hash__ already uses _is_pk_set() and raises for an unset PK.

The XML serializer has the same leftover (if obj.pk is not None) and emits pk='["None", "None"]' for an unsaved composite instance instead of omitting the attribute.

Proposed fix: use _is_pk_set() in both places, matching __hash__ and the rest of the #373 conversion.

Related: #18864, #18250, #373.

PR here: ​https://github.com/django/django/pull/21861

[Jacob Walls]

Small suggestion to move the release note and split into two commits, otherwise looks g2g.

[Jacob Walls <jacobtylerwalls@…>]

In 8d1c673:

Fixed #37309 -- Used _is_pk_set() in Model.eq.

Composite and db_default primary keys are not None when unset, so the
old pk is None check treated distinct unsaved instances as equal.

[Jacob Walls <jacobtylerwalls@…>]

In 6126f5d:

Refs #37309 -- Used _is_pk_set() in the XML serializer.

Unsaved composite and db_default primary keys are not None, so the old
pk is None check emitted a dummy pk attribute in XML dumps.