- Fixed a few edge cases which arise in the so-called "row switch"

scenario, where an INSERT/DELETE can be turned into an UPDATE.
In this situation, a many-to-one relationship set to None, or
in some cases a scalar attribute set to None, may not be detected
as a net change in value, and therefore the UPDATE would not reset
what was on the previous row.   This is due to some as-yet
unresovled side effects of the way attribute history works in terms
of implicitly assuming None isn't really a "change" for a previously
un-set attribute.  See also :ticket:`3061`. fixes #3060

1 files changed, 9 insertions, 4 deletions

diff --git a/lib/sqlalchemy/orm/dependency.py b/lib/sqlalchemy/orm/dependency.py
index 34a2af391..0d5a4f909 100644
--- a/lib/sqlalchemy/orm/dependency.py
+++ b/lib/sqlalchemy/orm/dependency.py
@@ -741,10 +741,15 @@ class ManyToOneDP(DependencyProcessor):
                                     self.key,
                                     attributes.PASSIVE_NO_INITIALIZE)
             if history:
-                for child in history.added:
-                    self._synchronize(state, child, None, False,
-                                            uowcommit, "add")
-
+                if history.added:
+                    for child in history.added:
+                        self._synchronize(state, child, None, False,
+                                                uowcommit, "add")
+                elif history.unchanged == [None]:
+                    # this is to appease the case where our row
+                    # here is in fact going to be a so-called "row switch",
+                    # where an INSERT becomes an UPDATE.  See #3060.
+                    self._synchronize(state, None, None, True, uowcommit)
                 if self.post_update:
                     self._post_update(state, uowcommit, history.sum())