Raise NotImplemenedError for association proxy __clause_element__

It's not possible right now to use an association proxy element as a plain
column expression to be SELECTed from or used in a SQL function.  An
informative error is now raised when this occurs.

Fixes: #5542
Change-Id: I334e767ebc0b56c1dccc4a1e5185b0435af77b93

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/sqlalchemy/ext/associationproxy.py b/lib/sqlalchemy/ext/associationproxy.py
index dd6217664..3ea77a952 100644
--- a/lib/sqlalchemy/ext/associationproxy.py
+++ b/lib/sqlalchemy/ext/associationproxy.py
@@ -421,6 +421,12 @@ class AssociationProxyInstance(object):
     def _comparator(self):
         return self._get_property().comparator
 
+    def __clause_element__(self):
+        raise NotImplementedError(
+            "The association proxy can't be used as a plain column "
+            "expression; it only works inside of a comparison expression"
+        )
+
     @classmethod
     def _cls_unwrap_target_assoc_proxy(cls, target_class, value_attr):
         attr = getattr(target_class, value_attr)