Establish future behavior for Session cascade backrefs, bind

The behavior of the :paramref:`_orm.relationship.cascade_backrefs` flag
will be reversed in 2.0 and set to ``False`` unconditionally, such that
backrefs don't cascade save-update operations from a forwards-assignment to
a backwards assignment.   A 2.0 deprecation warning is emitted when the
parameter is left at its default of ``True`` at the point at which such a
cascade operation actually takes place.   The new behavior can be
established as always by setting the flag to ``False`` on a specific
:func:`_orm.relationship`, or more generally can be set up across the board
by setting the the :paramref:`_orm.Session.future` flag to True.

Additionally in the interests of expediency, this commit will also
move Session away from making use of bound metadata if the future=True
flag is set.   An application that sets future=True should ideally
have to change as little else as possible for full 2.0 behavior.

Fixes: #5150
Change-Id: I490d1d61f09c62ffc2de983208aeed25dfe48aec

1 files changed, 8 insertions, 7 deletions

diff --git a/lib/sqlalchemy/testing/fixtures.py b/lib/sqlalchemy/testing/fixtures.py
index 1eac76598..1583147d4 100644
--- a/lib/sqlalchemy/testing/fixtures.py
+++ b/lib/sqlalchemy/testing/fixtures.py
@@ -293,13 +293,14 @@ class TablesTest(TestBase):
                 continue
             if table not in headers:
                 continue
-            cls.bind.execute(
-                table.insert(),
-                [
-                    dict(zip(headers[table], column_values))
-                    for column_values in rows[table]
-                ],
-            )
+            with cls.bind.begin() as conn:
+                conn.execute(
+                    table.insert(),
+                    [
+                        dict(zip(headers[table], column_values))
+                        for column_values in rows[table]
+                    ],
+                )
 
 
 class RemovesEvents(object):