descriptive error message raised when string-based relation() expressions inadvertently mistake a PropertyLoader for a ColumnLoader property

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/sqlalchemy/ext/declarative.py b/lib/sqlalchemy/ext/declarative.py
index 2a6facb55..8a58ca25d 100644
--- a/lib/sqlalchemy/ext/declarative.py
+++ b/lib/sqlalchemy/ext/declarative.py
@@ -328,7 +328,10 @@ class _GetColumns(object):
         if not mapper:
             return getattr(self.cls, key)
         else:
-            return mapper.get_property(key).columns[0]
+            prop = mapper.get_property(key)
+            if not isinstance(prop, ColumnProperty):
+                raise exceptions.InvalidRequestError("Property %r is not an instance of ColumnProperty (i.e. does not correspnd directly to a Column)." % key)
+            return prop.columns[0]
 
 
 def _deferred_relation(cls, prop):