Refs #29260 -- Doc'd Model.save() behavior change in Django 3.0.

docs/ref/models/instances.txt

@@ -490,6 +490,12 @@ which returns ``NULL``. In such cases it is possible to revert to the old
 algorithm by setting the :attr:`~django.db.models.Options.select_on_save`
 option to ``True``.
 
+.. versionchanged:: 3.0
+
+    ``Model.save()`` no longer attempts to find a row when saving a new
+    ``Model`` instance and a default value for the primary key is provided, and
+    always executes an ``INSERT``.
+
 .. _ref-models-force-insert:
 
 Forcing an INSERT or UPDATE

docs/releases/3.0.txt

@@ -372,6 +372,26 @@ Tests
 Backwards incompatible changes in 3.0
 =====================================
 
+``Model.save()`` when providing a default for the primary key
+-------------------------------------------------------------
+
+:meth:`.Model.save` no longer attempts to find a row when saving a new
+``Model`` instance and a default value for the primary key is provided, and
+always performs a single ``INSERT`` query. In older Django versions,
+``Model.save()`` performed either an ``INSERT`` or an ``UPDATE`` based on
+whether or not the row exists.
+
+This makes calling ``Model.save()`` while providing a default primary key value
+equivalent to passing :ref:`force_insert=True <ref-models-force-insert>` to
+model's ``save()``. Attempts to use a new ``Model`` instance to update an
+existing row will result in an ``IntegrityError``.
+
+In order to update an existing model for a specific primary key value use the
+:meth:`~django.db.models.query.QuerySet.update_or_create` method instead. For
+example::
+
+    >>> MyModel.objects.update_or_create(pk=existing_pk, defaults={'name': 'new name'})
+
 Database backend API
 --------------------