adjusted "blank out primary key" rule to check for "allow_null_pks" on target mapper. this was revealed by

recent attributes.py change in r3695 that causes a value of "None" to register as part of the attribute history's
added_items() collection (i.e. since AttributeHistory compares against NO_VALUE instead of None).

3 files changed, 35 insertions, 2 deletions

diff --git a/CHANGES b/CHANGES
index e8b94b21c..b84955746 100644
--- a/CHANGES
+++ b/CHANGES
@@ -87,7 +87,7 @@ CHANGES
   - Deferred column attributes no longer trigger a load operation when the
     attribute is assigned to.  In those cases, the newly assigned value
     will be present in the flushes' UPDATE statement unconditionally.
-
+    
   - Fixed a truncation error when re-assigning a subset of a collection
     (obj.relation = obj.relation[1:]) [ticket:834]
 
diff --git a/lib/sqlalchemy/orm/sync.py b/lib/sqlalchemy/orm/sync.py
index d85842861..5b5a9e43b 100644
--- a/lib/sqlalchemy/orm/sync.py
+++ b/lib/sqlalchemy/orm/sync.py
@@ -118,7 +118,7 @@ class SyncRule(object):
         try:
             return self._dest_primary_key
         except AttributeError:
-            self._dest_primary_key = self.dest_mapper is not None and self.dest_column in self.dest_mapper.pks_by_table[self.dest_column.table]
+            self._dest_primary_key = self.dest_mapper is not None and self.dest_column in self.dest_mapper.pks_by_table[self.dest_column.table] and not self.dest_mapper.allow_null_pks
             return self._dest_primary_key
 
     def execute(self, source, dest, obj, child, clearkeys):
diff --git a/test/orm/relationships.py b/test/orm/relationships.py
index d21121e18..282d1cafc 100644
--- a/test/orm/relationships.py
+++ b/test/orm/relationships.py
@@ -400,7 +400,40 @@ class RelationTest4(ORMTest):
             assert False
         except exceptions.AssertionError, e:
             assert str(e).startswith("Dependency rule tried to blank-out primary key column 'B.id' on instance ")
+            
+    def test_no_nullPK_sBtoA(self):
+        class A(object):pass
+        class B(object):pass
+        mapper(B, tableB, properties={
+            'a':relation(A, cascade="save-update")
+        })
+        mapper(A, tableA)
+        b1 = B()
+        b1.a = None
+        sess = create_session()
+        sess.save(b1)
+        try:
+            # this raises an error as of r3695.  in that rev, the attributes package was modified so that a 
+            # setting of "None" shows up as a change, which in turn fires off dependency.py and then triggers
+            # the rule.
+            sess.flush()
+            assert False
+        except exceptions.AssertionError, e:
+            assert str(e).startswith("Dependency rule tried to blank-out primary key column 'B.id' on instance ")
 
+    def test_nullPKsOK_sBtoA(self):
+        class A(object):pass
+        class B(object):pass
+        mapper(B, tableB, properties={
+            'a':relation(A, cascade="save-update")
+        }, allow_null_pks=True)
+        mapper(A, tableA)
+        b1 = B()
+        b1.a = None
+        sess = create_session()
+        sess.save(b1)
+        sess.flush()
+        
     def test_delete_cascade_BtoA(self):
         """test that the 'blank the PK' error doesnt get raised when the child is to be deleted as part of a 
         cascade"""