deferred property, checks for NULL primary key components and returns None

2 files changed, 7 insertions, 1 deletions

diff --git a/lib/sqlalchemy/mapping/properties.py b/lib/sqlalchemy/mapping/properties.py
index f3862ddd9..922aab453 100644
--- a/lib/sqlalchemy/mapping/properties.py
+++ b/lib/sqlalchemy/mapping/properties.py
@@ -80,7 +80,10 @@ class DeferredColumnProperty(ColumnProperty):
         def lazyload():
             clause = sql.and_()
             for primary_key in self.parent.pks_by_table[self.parent.primarytable]:
-                clause.clauses.append(primary_key == self.parent._getattrbycolumn(instance, primary_key))
+                attr = self.parent._getattrbycolumn(instance, primary_key)
+                if not attr:
+                    return None
+                clause.clauses.append(primary_key == attr)
             return sql.select([self.parent.table.c[self.key]], clause).scalar()
         return lazyload
 
diff --git a/test/mapper.py b/test/mapper.py
index 51751de6f..bcc6de098 100644
--- a/test/mapper.py
+++ b/test/mapper.py
@@ -184,6 +184,9 @@ class DeferredTest(MapperSuperTest):
             'description':deferred(orders.c.description)
         })
         
+        o = Order()
+        self.assert_(o.description is None)
+        
         def go():
             l = m.select()
             o2 = l[2]