Consider merge key with (None, ) as non-persistent

Fixed bug in :meth:`.Session.merge` where objects in a collection that had
the primary key attribute set to ``None`` for a key that is  typically
autoincrementing would be considered to be a database-persisted key for
part of the internal deduplication process, causing only one object to
actually be inserted in the database.

Change-Id: I0a6e00043be0b2979cda33740e1be3b430ecf8c7
Fixes: #4056
(cherry picked from commit 5243341ed886e10a0d3f7fef8ae3d071e0ffdcf0)

1 files changed, 5 insertions, 5 deletions

diff --git a/lib/sqlalchemy/orm/session.py b/lib/sqlalchemy/orm/session.py
index 6ecef17f1..5488c3031 100644
--- a/lib/sqlalchemy/orm/session.py
+++ b/lib/sqlalchemy/orm/session.py
@@ -1912,7 +1912,10 @@ class Session(_SessionClassMethods):
                     "all changes on mapped instances before merging with "
                     "load=False.")
             key = mapper._identity_key_from_state(state)
-            key_is_persistent = attributes.NEVER_SET not in key[1]
+            key_is_persistent = attributes.NEVER_SET not in key[1] and (
+                not _none_set.intersection(key[1]) or
+                (mapper.allow_partial_pks and not _none_set.issuperset(key[1]))
+            )
         else:
             key_is_persistent = True
 
@@ -1933,10 +1936,7 @@ class Session(_SessionClassMethods):
             self._update_impl(merged_state)
             new_instance = True
 
-        elif key_is_persistent and (
-            not _none_set.intersection(key[1]) or
-            (mapper.allow_partial_pks and
-             not _none_set.issuperset(key[1]))):
+        elif key_is_persistent:
             merged = self.query(mapper.class_).get(key[1])
         else:
             merged = None