[2.0.x] Refs #28010 -- Allowed reverse related fields in SELECT FOR UPDATE .. OF.

Thanks Adam Chidlow for polishing the patch.

Backport of 03049fb8d96ccd1f1ed0285486103542de42faba from master

1 files changed, 13 insertions, 0 deletions

diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt
index 81caf97aa7..0b6e53ed1b 100644
--- a/docs/ref/models/querysets.txt
+++ b/docs/ref/models/querysets.txt
@@ -1650,6 +1650,19 @@ 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.
 
+You can't use ``select_for_update()`` on nullable relations::
+
+    >>> Person.objects.select_related('hometown').select_for_update()
+    Traceback (most recent call last):
+    ...
+    django.db.utils.NotSupportedError: FOR UPDATE cannot be applied to the nullable side of an outer join
+
+To avoid that restriction, you can exclude null objects if you don't care about
+them::
+
+    >>> Person.objects.select_related('hometown').select_for_update().exclude(hometown=None)
+    <QuerySet [<Person: ...)>, ...]>
+
 Currently, the ``postgresql``, ``oracle``, and ``mysql`` database
 backends support ``select_for_update()``. However, MySQL doesn't support the
 ``nowait``, ``skip_locked``, and ``of`` arguments.