Do not use STI mechanisms when migrating pipeline stages

lib/gitlab/background_migration/migrate_build_stage.rb

@@ -12,6 +12,7 @@ module Gitlab
 
         class Build < ActiveRecord::Base
           self.table_name = 'ci_builds'
+          self.inheritance_column = :_type_disabled
 
           def ensure_stage!(attempts: 2)
             find_stage || create_stage!

spec/lib/gitlab/background_migration/migrate_build_stage_spec.rb

@@ -51,4 +51,20 @@ describe Gitlab::BackgroundMigration::MigrateBuildStage, :migration, schema: 201
     expect { described_class.new.perform(1, 6) }
       .to raise_error ActiveRecord::RecordNotUnique
   end
+
+  context 'when invalid class can be loaded due to single table inheritance' do
+    let(:commit_status) do
+      jobs.create!(id: 7, commit_id: 1, project_id: 123, stage_idx: 4,
+                   stage: 'post-deploy', status: :failed)
+    end
+
+    before do
+      commit_status.update_column(:type, 'SomeClass')
+    end
+
+    it 'does ignore single table inheritance type' do
+      expect { described_class.new.perform(1, 7) }.not_to raise_error
+      expect(jobs.find(7)).to have_attributes(stage_id: (a_value > 0))
+    end
+  end
 end