Add migration to remove orphaned notification settings

db/migrate/20170418103908_delete_orphan_notification_settings.rb

+class DeleteOrphanNotificationSettings < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    execute("DELETE FROM notification_settings WHERE EXISTS (SELECT true FROM (#{orphan_notification_settings}) AS ns WHERE ns.id = notification_settings.id)")
+  end
+
+  def down
+    # This is a no-op method to make the migration reversible.
+    # If someone is trying to rollback for other reasons, we should not throw an Exception.
+    # raise ActiveRecord::IrreversibleMigration
+  end
+
+  def orphan_notification_settings
+    <<-SQL
+      SELECT notification_settings.id
+      FROM   notification_settings
+             LEFT OUTER JOIN namespaces
+                          ON namespaces.id = notification_settings.source_id
+      WHERE  notification_settings.source_type = 'Namespace'
+             AND namespaces.id IS NULL
+    SQL
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170408033905) do
+ActiveRecord::Schema.define(version: 20170418103908) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"