Remove use of Session.begin.subtransactions flag

Resolve the following RemovedIn20Warning warning:

  The Session.begin.subtransactions flag is deprecated and will be
  removed in SQLAlchemy version 2.0.  See the documentation at
  session_subtransactions for background on a compatible alternative
  pattern.

Change-Id: Ib2537bae77861ee60d17a48a72c57b88e043553e
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

oslo_db/sqlalchemy/models.py

@@ -34,18 +34,8 @@ class ModelBase(object):
 
     def save(self, session):
         """Save this object."""
-
-        # NOTE(boris-42): This part of code should be look like:
-        #                       session.add(self)
-        #                       session.flush()
-        #                 But there is a bug in sqlalchemy and eventlet that
-        #                 raises NoneType exception if there is no running
-        #                 transaction and rollback is called. As long as
-        #                 sqlalchemy has this bug we have to create transaction
-        #                 explicitly.
-        with session.begin(subtransactions=True):
-            session.add(self)
-            session.flush()
+        session.add(self)
+        session.flush()
 
     def __setitem__(self, key, value):
         setattr(self, key, value)

oslo_db/tests/fixtures.py

@@ -42,11 +42,6 @@ class WarningsFixture(fixtures.Fixture):
             message=r'The Session.autocommit parameter is deprecated .*',
             category=sqla_exc.SADeprecationWarning)
 
-        warnings.filterwarnings(
-            'once',
-            message=r'The Session.begin.subtransactions flag is deprecated .*',
-            category=sqla_exc.SADeprecationWarning)
-
         warnings.filterwarnings(
             'once',
             message=r'Calling \.begin\(\) when a transaction is already .*',