Check for non-mapped property in synonym

An :class:`.InvalidRequestError` is raised when a :func:`.synonym`
is used against an attribute that is not against a :class:`.MapperProperty`,
such as an association proxy.  Previously, a recursion overflow would
occur trying to locate non-existent attributes.

Change-Id: If2ce38c429a69951df4c94b71b74edbd59d775e3
Fixes: #4067

1 files changed, 12 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/descriptor_props.py b/lib/sqlalchemy/orm/descriptor_props.py
index 9afdbf693..b9f016b42 100644
--- a/lib/sqlalchemy/orm/descriptor_props.py
+++ b/lib/sqlalchemy/orm/descriptor_props.py
@@ -594,7 +594,18 @@ class SynonymProperty(DescriptorProperty):
 
     @util.memoized_property
     def _proxied_property(self):
-        return getattr(self.parent.class_, self.name).property
+        attr = getattr(self.parent.class_, self.name)
+        if not hasattr(attr, 'property') or not \
+                isinstance(attr.property, MapperProperty):
+            raise sa_exc.InvalidRequestError(
+                """synonym() attribute "%s.%s" only supports """
+                """ORM mapped attributes, got %r""" % (
+                    self.parent.class_.__name__,
+                    self.name,
+                    attr
+                )
+            )
+        return attr.property
 
     def _comparator_factory(self, mapper):
         prop = self._proxied_property