From 25b6e811e692d20495ea6bed93552a22a28f9d92 Mon Sep 17 00:00:00 2001
From: Mike Bayer <mike_mp@zzzcomputing.com>
Date: Fri, 4 May 2012 19:27:52 -0400
Subject: [PATCH]   - [bug] Fixed issue in unit of work     whereby setting a
 non-None self-referential     many-to-one relationship to None     would fail
 to persist the change if the     former value was not already loaded.    
 [ticket:2477].

---
 CHANGES                          |  7 +++++++
 lib/sqlalchemy/orm/attributes.py |  2 +-
 test/orm/test_unitofworkv2.py    | 25 +++++++++++++++++++++++++
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/CHANGES b/CHANGES
index fde7c681d6..d29e6287ab 100644
--- a/CHANGES
+++ b/CHANGES
@@ -6,6 +6,13 @@ CHANGES
 0.7.7
 =====
 - orm
+  - [bug] Fixed issue in unit of work
+    whereby setting a non-None self-referential
+    many-to-one relationship to None
+    would fail to persist the change if the
+    former value was not already loaded.
+    [ticket:2477].
+
   - [feature] Added prefix_with() method
     to Query, calls upon select().prefix_with()
     to allow placement of MySQL SELECT
diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index 3b4f18b310..a7e1824a65 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -653,7 +653,7 @@ class ScalarObjectAttributeImpl(ScalarAttributeImpl):
             if current is not None:
                 ret = [(instance_state(current), current)]
             else:
-                ret = []
+                ret = [(None, None)]
 
             if self.key in state.committed_state:
                 original = state.committed_state[self.key]
diff --git a/test/orm/test_unitofworkv2.py b/test/orm/test_unitofworkv2.py
index d7e2150db5..cfd36e696d 100644
--- a/test/orm/test_unitofworkv2.py
+++ b/test/orm/test_unitofworkv2.py
@@ -777,6 +777,31 @@ class SingleCycleTest(UOWTest):
                         lambda ctx: {'id':n1.id})
         )
 
+    def test_many_to_one_set_null_unloaded(self):
+        Node, nodes = self.classes.Node, self.tables.nodes
+
+        mapper(Node, nodes, properties={
+            'parent':relationship(Node, remote_side=nodes.c.id)
+        })
+        sess = create_session()
+        n1 = Node(data='n1')
+        n2 = Node(data='n2', parent=n1)
+        sess.add_all([n1, n2])
+        sess.flush()
+        sess.close()
+
+        n2 = sess.query(Node).filter_by(data='n2').one()
+        n2.parent = None
+        self.assert_sql_execution(
+            testing.db,
+            sess.flush,
+            CompiledSQL(
+                "UPDATE nodes SET parent_id=:parent_id WHERE "
+                "nodes.id = :nodes_id",
+                lambda ctx: {"parent_id":None, "nodes_id":n2.id}
+            )
+        )
+
     def test_cycle_rowswitch(self):
         Node, nodes = self.classes.Node, self.tables.nodes
 
-- 
2.47.2