Fix nested index_property setter when there is no container value

Fixed bug in new :mod:`sqlalchemy.ext.indexable` extension
where setting of a property that itself refers to another property
would fail.

Fixes: #3901
Change-Id: I203a66117e2399afee11a34f43f0e93adfc6d571

1 files changed, 4 insertions, 2 deletions

diff --git a/lib/sqlalchemy/ext/indexable.py b/lib/sqlalchemy/ext/indexable.py
index 8298a65a9..b1ce12923 100644
--- a/lib/sqlalchemy/ext/indexable.py
+++ b/lib/sqlalchemy/ext/indexable.py
@@ -227,6 +227,7 @@ The above query will render::
 """
 from __future__ import absolute_import
 
+from sqlalchemy import inspect
 from ..orm.attributes import flag_modified
 from ..ext.hybrid import hybrid_property
 
@@ -318,13 +319,14 @@ class index_property(hybrid_property):  # noqa
 
     def fset(self, instance, value):
         attr_name = self.attr_name
-        column_value = getattr(instance, attr_name)
+        column_value = getattr(instance, attr_name, None)
         if column_value is None:
             column_value = self.datatype()
             setattr(instance, attr_name, column_value)
         column_value[self.index] = value
         setattr(instance, attr_name, column_value)
-        flag_modified(instance, attr_name)
+        if attr_name in inspect(instance).mapper.attrs:
+            flag_modified(instance, attr_name)
 
     def fdel(self, instance):
         attr_name = self.attr_name