[2.2.x] Fixed #30953 -- Made select_for_update() lock queryset's model when using "self" with multi-table inheritance.

Thanks Abhijeet Viswa for the report and initial patch.
Backport of 0107e3d1058f653f66032f7fd3a0bd61e96bf782 from master

2 files changed, 14 insertions, 0 deletions

diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt
index 3ccd5958fd..f23d4bc598 100644
--- a/docs/ref/models/querysets.txt
+++ b/docs/ref/models/querysets.txt
@@ -1696,6 +1696,14 @@ specify the related objects you want to lock in ``select_for_update(of=(...))``
 using the same fields syntax as :meth:`select_related`. Use the value ``'self'``
 to refer to the queryset's model.
 
+.. admonition:: Lock parents models in ``select_for_update(of=(...))``
+
+    If you want to lock parents models when using :ref:`multi-table inheritance
+    <multi-table-inheritance>`, you must specify parent link fields (by default
+    ``<parent_model_name>_ptr``) in the ``of`` argument. For example::
+
+        Restaurant.objects.select_for_update(of=('self', 'place_ptr'))
+
 You can't use ``select_for_update()`` on nullable relations::
 
     >>> Person.objects.select_related('hometown').select_for_update()
diff --git a/docs/releases/2.2.8.txt b/docs/releases/2.2.8.txt
index 4d8f9869c5..3c5eb5c754 100644
--- a/docs/releases/2.2.8.txt
+++ b/docs/releases/2.2.8.txt
@@ -17,3 +17,9 @@ Bugfixes
 * Fixed a regression in Django 2.2.1 that caused a crash when migrating
   permissions for proxy models with a multiple database setup if the
   ``default`` entry was empty (:ticket:`31021`).
+
+* Fixed a data loss possibility in the
+  :meth:`~django.db.models.query.QuerySet.select_for_update()`. When using
+  ``'self'`` in the ``of`` argument with :ref:`multi-table inheritance
+  <multi-table-inheritance>`, a parent model was locked instead of the
+  queryset's model (:ticket:`30953`).