[Rails5] Disable inheritance_column for...

[Rails5] Disable inheritance_column for Gitlab::Database::RenameReservedPathsMigration::V1::MigrationClasses::Namespace

changelogs/unreleased/blackst0ne-rails5-invalid-single-table-inheritance-type-group-is-not-a-subclass-of-namespace.yml

+---
+title: "[Rails5] Invalid single-table inheritance type: Group is not a subclass of
+  Namespace"
+merge_request: 19918
+author: "@blackst0ne"
+type: fixed

lib/gitlab/database/rename_reserved_paths_migration/v1/migration_classes.rb

@@ -37,6 +37,7 @@ module Gitlab
           class Namespace < ActiveRecord::Base
             include MigrationClasses::Routable
             self.table_name = 'namespaces'
+            self.inheritance_column = :_type_disabled
             belongs_to :parent,
                        class_name: "#{MigrationClasses.name}::Namespace"
             has_one :route, as: :source