- An exception is raised in the unusual case that an

append or similar event on a collection occurs after
the parent object has been dereferenced, which
prevents the parent from being marked as "dirty"
in the session.  Will commit as a warning in 0.6.
[ticket:2046]

2 files changed, 10 insertions, 2 deletions

diff --git a/lib/sqlalchemy/orm/exc.py b/lib/sqlalchemy/orm/exc.py
index b86e5c7c3..3bfb2708c 100644
--- a/lib/sqlalchemy/orm/exc.py
+++ b/lib/sqlalchemy/orm/exc.py
@@ -40,6 +40,9 @@ class FlushError(sa.exc.SQLAlchemyError):
 class UnmappedError(sa.exc.InvalidRequestError):
     """Base for exceptions that involve expected mappings not present."""
 
+class ObjectDereferencedError(sa.exc.SQLAlchemyError):
+    """An operation cannot complete due to an object being garbage collected."""
+
 class DetachedInstanceError(sa.exc.SQLAlchemyError):
     """An attempt to access unloaded attributes on a 
     mapped instance that is detached."""
diff --git a/lib/sqlalchemy/orm/state.py b/lib/sqlalchemy/orm/state.py
index 7a93b5cb0..6b410a868 100644
--- a/lib/sqlalchemy/orm/state.py
+++ b/lib/sqlalchemy/orm/state.py
@@ -15,7 +15,8 @@ from sqlalchemy.util import EMPTY_SET
 import weakref
 from sqlalchemy import util
 
-from sqlalchemy.orm import exc as orm_exc, attributes, interfaces
+from sqlalchemy.orm import exc as orm_exc, attributes, interfaces,\
+        util as orm_util
 from sqlalchemy.orm.attributes import PASSIVE_OFF, PASSIVE_NO_RESULT, \
     PASSIVE_NO_FETCH, NEVER_SET, ATTR_WAS_SET, NO_VALUE
 
@@ -351,7 +352,11 @@ class InstanceState(object):
                 instance_dict._modified.add(self)
 
             self._strong_obj = self.obj()
-
+            if self._strong_obj is None:
+                raise orm_exc.ObjectDereferencedError(
+                        "Can't emit change event for attribute '%s' - parent object "
+                        "of type %s has been garbage collected." 
+                        % (self.manager[attr.key], orm_util.state_class_str(self)))
             self.modified = True
 
     def commit(self, dict_, keys):