Check for supports_execution at ClauseElement base

Raise a more descriptive exception / message when ClauseElement
or non-SQLAlchemy objects that are not "executable" are erroneously
passed to ``.execute()``; a new exception ObjectNotExecutableError
is raised consistently in all cases.

Change-Id: I2dd393121e2c7e5b6b9e40286a2f25670876e8e4
Fixes: #3786

1 files changed, 4 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index e277b28a4..75d5368d5 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -259,7 +259,10 @@ class ClauseElement(Visitable):
             return self
 
     def _execute_on_connection(self, connection, multiparams, params):
-        return connection._execute_clauseelement(self, multiparams, params)
+        if self.supports_execution:
+            return connection._execute_clauseelement(self, multiparams, params)
+        else:
+            raise exc.ObjectNotExecutableError(self)
 
     def unique_params(self, *optionaldict, **kwargs):
         """Return a copy with :func:`bindparam()` elements replaced.