db: Remove use of 'as_scalar()'

Resolve the following RemovedIn20Warning warning:

  The SelectBase.as_scalar() method is deprecated and will be removed in
  a future release.

Change-Id: If1179e8d94cce54cdc8ac1a30eb63a3bf943042d
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

cinder/db/sqlalchemy/api.py

@@ -3361,8 +3361,15 @@ def volume_qos_allows_retype(new_vol_type):
     specifies anything other than the back-end in any of the 2 volume_types.
     """
     # Query to get the qos of the volume type new_vol_type
-    q = sql.select(models.VolumeType.qos_specs_id).where(
-        and_(~models.VolumeType.deleted, models.VolumeType.id == new_vol_type)
+    q = (
+        sql.select(models.VolumeType.qos_specs_id)
+        .where(
+            and_(
+                ~models.VolumeType.deleted,
+                models.VolumeType.id == new_vol_type,
+            )
+        )
+        .scalar_subquery()
     )
     # Construct the filter to check qos when volume is 'in-use'
     return or_(
@@ -3373,7 +3380,7 @@ def volume_qos_allows_retype(new_vol_type):
             and_(
                 ~models.VolumeType.deleted,
                 models.VolumeType.id == models.Volume.volume_type_id,
-                models.VolumeType.qos_specs_id == q.as_scalar(),
+                models.VolumeType.qos_specs_id == q,
             )
         ),
         # Or they are different specs but they are handled by the backend or

cinder/tests/fixtures.py

@@ -197,13 +197,6 @@ class WarningsFixture(fixtures.Fixture):
             category=sqla_exc.SADeprecationWarning,
         )
 
-        warnings.filterwarnings(
-            'ignore',
-            module='cinder',
-            message=r'The SelectBase.as_scalar\(\) method is deprecated ',
-            category=sqla_exc.SADeprecationWarning,
-        )
-
         warnings.filterwarnings(
             'ignore',
             module='cinder',