[Rails5] Fix 'Invalid single-table inheritance type: Group is not a subclass...

[Rails5] Fix 'Invalid single-table inheritance type: Group is not a subclass of Gitlab::BackgroundMigration::FixCrossProjectLabelLinks::Namespace'

changelogs/unreleased/48976-rails5-invalid-single-table-inheritance-type-group-is-not-a-subclass-of-gitlab-backgroundmigration-fixcrossprojectlabellinks-namespace.yml

+---
+title: "[Rails5] Fix 'Invalid single-table inheritance type: Group is not a subclass
+  of Gitlab::BackgroundMigration::FixCrossProjectLabelLinks::Namespace'"
+merge_request: 20462
+author: "@blackst0ne"
+type: fixed

lib/gitlab/background_migration/fix_cross_project_label_links.rb

@@ -11,6 +11,7 @@ module Gitlab
       end
 
       class Label < ActiveRecord::Base
+        self.inheritance_column = :_type_disabled
         self.table_name = 'labels'
       end
 
@@ -27,6 +28,7 @@ module Gitlab
       end
 
       class Namespace < ActiveRecord::Base
+        self.inheritance_column = :_type_disabled
         self.table_name = 'namespaces'
 
         def self.groups_with_descendants_ids(start_id, stop_id)