Fixed #22967 -- Made Model._do_update consistent

Made _do_update behave more strictly according to its docs,
including a corner case when specific concurent updates are
executed and select_on_save is set.

1 files changed, 8 insertions, 2 deletions

diff --git a/django/db/models/base.py b/django/db/models/base.py
index d416b65ccf..f2404d40a5 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -757,8 +757,14 @@ class Model(six.with_metaclass(ModelBase)):
             return update_fields is not None or filtered.exists()
         if self._meta.select_on_save and not forced_update:
             if filtered.exists():
-                filtered._update(values)
-                return True
+                # It may happen that the object is deleted from the DB right after
+                # this check, causing the subsequent UPDATE to return zero matching
+                # rows. The same result can occur in some rare cases when the
+                # database returns zero despite the UPDATE being executed
+                # successfully (a row is matched and updated). In order to
+                # distinguish these two cases, the object's existence in the
+                # database is again checked for if the UPDATE query returns 0.
+                return filtered._update(values) > 0 or filtered.exists()
             else:
                 return False
         return filtered._update(values) > 0