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, 3 insertions, 3 deletions

diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index e364b2e7f..98a96fd56 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -71,9 +71,6 @@ class SchemaItem(SchemaEventTarget, visitors.Visitable):
 
     __visit_name__ = 'schema_item'
 
-    def _execute_on_connection(self, connection, multiparams, params):
-        return connection._execute_default(self, multiparams, params)
-
     def _init_items(self, *args):
         """Initialize the list of child items for this SchemaItem."""
 
@@ -1941,6 +1938,9 @@ class DefaultGenerator(_NotAColumnExpr, SchemaItem):
             bind = _bind_or_error(self)
         return bind._execute_default(self, **kwargs)
 
+    def _execute_on_connection(self, connection, multiparams, params):
+        return connection._execute_default(self, multiparams, params)
+
     @property
     def bind(self):
         """Return the connectable associated with this default."""