- 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, 12 insertions, 5 deletions

diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index bf7dab4e7..09d6e988d 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -568,7 +568,7 @@ class AttributeImpl(object):
             # if history present, don't load
             key = self.key
             if key not in state.committed_state or \
-                state.committed_state[key] is NEVER_SET:
+                    state.committed_state[key] is NEVER_SET:
                 if not passive & CALLABLES_OK:
                     return PASSIVE_NO_RESULT
 
@@ -763,6 +763,13 @@ class ScalarObjectAttributeImpl(ScalarAttributeImpl):
         if self.dispatch._active_history:
             old = self.get(state, dict_, passive=PASSIVE_ONLY_PERSISTENT | NO_AUTOFLUSH)
         else:
+            # would like to call with PASSIVE_NO_FETCH ^ INIT_OK.  However,
+            # we have a long-standing behavior that a "get()" on never set
+            # should implicitly set the value to None.  Leaving INIT_OK
+            # set here means we are consistent whether or not we did a get
+            # first.
+            # see test_use_object_set_None vs. test_use_object_get_first_set_None
+            # in test_attributes.py
             old = self.get(state, dict_, passive=PASSIVE_NO_FETCH)
 
         if check_old is not None and \
@@ -777,6 +784,7 @@ class ScalarObjectAttributeImpl(ScalarAttributeImpl):
                    state_str(state),
                    self.key
                 ))
+
         value = self.fire_replace_event(state, dict_, value, old, initiator)
         dict_[self.key] = value
 
@@ -793,8 +801,7 @@ class ScalarObjectAttributeImpl(ScalarAttributeImpl):
     def fire_replace_event(self, state, dict_, value, previous, initiator):
         if self.trackparent:
             if (previous is not value and
-                previous is not None and
-                previous is not PASSIVE_NO_RESULT):
+                    previous not in (None, PASSIVE_NO_RESULT, NEVER_SET)):
                 self.sethasparent(instance_state(previous), state, False)
 
         for fn in self.dispatch.set:
@@ -1080,7 +1087,7 @@ def backref_listeners(attribute, key, uselist):
     def emit_backref_from_scalar_set_event(state, child, oldchild, initiator):
         if oldchild is child:
             return child
-        if oldchild is not None and oldchild is not PASSIVE_NO_RESULT:
+        if oldchild is not None and oldchild not in (PASSIVE_NO_RESULT, NEVER_SET):
             # With lazy=None, there's no guarantee that the full collection is
             # present when updating via a backref.
             old_state, old_dict = instance_state(oldchild),\
@@ -1208,7 +1215,7 @@ class History(History):
 
         return not bool(
                         (self.added or self.deleted)
-                        or self.unchanged and self.unchanged != [None]
+                        or self.unchanged
                     )
 
     def sum(self):