Fix notification method type DB schema migration

The Stein release does away with the concept of built in notifications,
in favour of treating all notification types equally.

This patch fixes an issue with the DB schema migration associated
with this change, which will fail if any notifications using
built-in notification types are configured at the time of the upgrade.

Story: 2006984
Task: 37746
Change-Id: I2e3f08edf1ab6aec526ad93d04effb91ddca600a

monasca_api/db/alembic/versions/26083b298bb7_remove_builtin_notification_types.py

@@ -22,6 +22,8 @@ from alembic import op
 import sqlalchemy as sa
 from sqlalchemy.sql import table
 
+_NM_BUILT_IN_TYPES = set(['EMAIL', 'WEBHOOK', 'PAGERDUTY'])
+
 # revision identifiers, used by Alembic.
 revision = '26083b298bb7'
 down_revision = 'f69cb3152a76'
@@ -34,13 +36,29 @@ _nm_types = table(
               sa.String(length=20),
               nullable=False))
 
+_nm = table(
+    'notification_method',
+    sa.Column('type',
+              sa.String(length=20),
+              nullable=False))
+
 
 def upgrade():
     # Built-in notification types have been removed. Here, we
-    # remove them and rely on monasca_notification to re-populate
-    # the table according to what is set in its config file.
+    # remove them (where not in use) and rely on Monasca Notification
+    # to re-populate the table according to what is set in its config file.
+
+    # Start by creating a set of all notification method types currently
+    # configured in the Monasca DB
+    connection = op.get_bind()
+    nm_types_configured = connection.execute(_nm.select()).fetchall()
+    nm_types_configured = set([nm_type[0] for nm_type in nm_types_configured])
+
+    # Remove all built in notification types which are currently *not*
+    # configured.
+    nm_types_to_remove = _NM_BUILT_IN_TYPES.difference(nm_types_configured)
     op.execute(_nm_types.delete().where(
-        _nm_types.c.name.in_(('EMAIL', 'WEBHOOK', 'PAGERDUTY'))))
+        _nm_types.c.name.in_(nm_types_to_remove)))
 
 
 def downgrade():

releasenotes/notes/fix-db-migration-issue-2006984-6676bd3a8a34c9ae.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    An issue with migrating the DB schema in the Stein release
+    which can cause an upgrade to fail. See `story
+    2006984 <https://storyboard.openstack.org/#!/story/2006984>`__