- [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].

3 files changed, 33 insertions, 1 deletions

diff --git a/CHANGES b/CHANGES
index 0c47aa66e..0be1dc951 100644
--- a/CHANGES
+++ b/CHANGES
@@ -71,6 +71,13 @@ CHANGES
     set of objects that weren't modified in 
     that sub-transaction.  [ticket:2452]
 
+  - [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]. Also in 0.7.7
+
   - [bug] Fixed bug in relationship comparisons
     whereby calling unimplemented methods like
     SomeClass.somerelationship.like() would
diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index 7625ccead..00d640066 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -627,7 +627,7 @@ 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 d7e2150db..cfd36e696 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