Fix oslo.db >= 15.0.0 compatibility

Minimal refactor of SQLAlchemy api module to be compatible with
oslo.db >= 15.0.0 where autocommit behaviour was dropped.

Closes-Bug: #2056181
Change-Id: I33be53f647faae2aad30a43c10980df950d5d7c2
(cherry picked from commit bc5922c684)

watcher/db/sqlalchemy/api.py

@@ -44,11 +44,7 @@ _FACADE = None
 def _create_facade_lazily():
     global _FACADE
     if _FACADE is None:
-        # FIXME(amoralej): Remove autocommit=True (and ideally use of
-        # LegacyEngineFacade) asap since it's not compatible with SQLAlchemy
-        # 2.0.
-        _FACADE = db_session.EngineFacade.from_config(CONF,
-                                                      autocommit=True)
+        _FACADE = db_session.EngineFacade.from_config(CONF)
     return _FACADE
 
 
@@ -252,15 +248,20 @@ class Connection(api.BaseConnection):
         return query
 
     def _create(self, model, values):
+        session = get_session()
+        with session.begin():
             obj = model()
             cleaned_values = {k: v for k, v in values.items()
                               if k not in self._get_relationships(model)}
             obj.update(cleaned_values)
-        obj.save()
+            obj.save(session=session)
+            session.commit()
         return obj
 
     def _get(self, context, model, fieldname, value, eager):
-        query = model_query(model)
+        session = get_session()
+        with session.begin():
+            query = model_query(model, session=session)
             if eager:
                 query = self._set_eager_options(model, query)