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

docs/ref/models/instances.txt

@@ -465,8 +465,9 @@ How Django knows to UPDATE vs. INSERT
 
 You may have noticed Django database objects use the same ``save()`` method
 for creating and changing objects. Django abstracts the need to use ``INSERT``
-or ``UPDATE`` SQL statements. Specifically, when you call ``save()``, Django
+or ``UPDATE`` SQL statements. Specifically, when you call ``save()`` and the
-follows this algorithm:
+object's primary key attribute does **not** define a
+:attr:`~django.db.models.Field.default`, Django follows this algorithm:
 
 * If the object's primary key attribute is set to a value that evaluates to
   ``True`` (i.e., a value other than ``None`` or the empty string), Django
@@ -475,6 +476,11 @@ follows this algorithm:
   didn't update anything (e.g. if primary key is set to a value that doesn't
   exist in the database), Django executes an ``INSERT``.
 
+If the object's primary key attribute defines a
+:attr:`~django.db.models.Field.default` then Django executes an ``UPDATE`` if
+it is an existing model instance and primary key is set to a value that exists
+in the database. Otherwise, Django executes an ``INSERT``.
+
 The one gotcha here is that you should be careful not to specify a primary-key
 value explicitly when saving new objects, if you cannot guarantee the
 primary-key value is unused. For more on this nuance, see `Explicitly specifying
@@ -490,6 +496,12 @@ 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 +372,27 @@ 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 or
+``QuerySet.filter(pk=…).update(…)`` instead. For example::
+
+    >>> MyModel.objects.update_or_create(pk=existing_pk, defaults={'name': 'new name'})
+    >>> MyModel.objects.filter(pk=existing_pk).update(name='new name')
+
 Database backend API
 --------------------