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, 20 insertions, 0 deletions

diff --git a/test/engine/test_execute.py b/test/engine/test_execute.py
index 8e553307f..49b29f7f2 100644
--- a/test/engine/test_execute.py
+++ b/test/engine/test_execute.py
@@ -306,6 +306,26 @@ class ExecuteTest(fixtures.TestBase):
         finally:
             conn.close()
 
+    def test_not_an_executable(self):
+        for obj in (
+            Table("foo", MetaData(), Column("x", Integer)),
+            Column('x', Integer),
+            tsa.and_(),
+            column('foo'),
+            tsa.and_().compile(),
+            column('foo').compile(),
+            MetaData(),
+            Integer(),
+            tsa.Index(name='foo'),
+            tsa.UniqueConstraint('x')
+        ):
+            with testing.db.connect() as conn:
+                assert_raises_message(
+                    tsa.exc.ObjectNotExecutableError,
+                    "Not an executable object",
+                    conn.execute, obj
+                )
+
     def test_stmt_exception_non_ascii(self):
         name = util.u('méil')
         with testing.db.connect() as conn: