db: Remove inplicit coercion of SELECTs

Resolve the following RemovedIn20Warning warning:

  Implicit coercion of SELECT and textual SELECT constructs into FROM
  clauses is deprecated; please call .subquery() on any Core select or
  ORM Query object in order to produce a subquery object.

This one was easy.

Change-Id: Ifeab2aa8cef7ad151d5d5f92937e90ab34b96e8a
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

nova/db/main/api.py

@@ -579,7 +579,7 @@ def _compute_node_select(context, filters=None, limit=None, marker=None):
     if filters is None:
         filters = {}
 
-    cn_tbl = sa.alias(models.ComputeNode.__table__, name='cn')
+    cn_tbl = models.ComputeNode.__table__.alias('cn')
     select = sa.select(cn_tbl)
 
     if context.read_deleted == "no":
@@ -926,7 +926,7 @@ def compute_node_statistics(context):
     engine = get_engine(context=context)
     services_tbl = models.Service.__table__
 
-    inner_sel = sa.alias(_compute_node_select(context), name='inner_sel')
+    inner_sel = _compute_node_select(context).alias('inner_sel')
 
     # TODO(sbauza): Remove the service_id filter in a later release
     # once we are sure that all compute nodes report the host field

nova/tests/fixtures/nova.py

@@ -885,12 +885,6 @@ class WarningsFixture(fixtures.Fixture):
             message=r'The Column.copy\(\) method is deprecated .*',
             category=sqla_exc.SADeprecationWarning)
 
-        warnings.filterwarnings(
-            'ignore',
-            module='nova',
-            message='Implicit coercion of SELECT and textual SELECT .*',
-            category=sqla_exc.SADeprecationWarning)
-
         self.addCleanup(self._reset_warning_filters)
 
     def _reset_warning_filters(self):