Perform stage_id ref backgound migration in bulks

db/post_migrate/20170628080858_migrate_stage_id_reference_in_background.rb

@@ -15,9 +15,9 @@ class MigrateStageIdReferenceInBackground < ActiveRecord::Migration
     Build.where(stage_id: nil)
       .in_batches(of: BATCH_SIZE) do |relation, index|
         schedule = index * 5.minutes
-        relation.each do |build|
-          BackgroundMigrationWorker.perform_at(schedule, MIGRATION, [build.id])
-        end
+        jobs = relation.pluck(:id).map { |id| [MIGRATION, [id]] }
+
+        BackgroundMigrationWorker.perform_bulk_in(schedule, jobs)
       end
   end