Find builds that require a migration in batches

db/post_migrate/20170628080858_migrate_stage_id_reference_in_background.rb

@@ -2,6 +2,8 @@ class MigrateStageIdReferenceInBackground < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   DOWNTIME = false
+  BATCH_SIZE = 10000
+  MIGRATION = 'MigrateBuildStageIdReference'.freeze
 
   disable_ddl_transaction!
 
@@ -10,9 +12,10 @@ class MigrateStageIdReferenceInBackground < ActiveRecord::Migration
   end
 
   def up
-    Build.find_each do |build|
-      BackgroundMigrationWorker
-        .perform_async('MigrateBuildStageIdReference', [build.id])
+    Build.find_in_batches(batch_size: BATCH_SIZE).with_index do |builds, batch|
+      migrations = builds.map { |build| [MIGRATION, [build.id]] }
+
+      BackgroundMigrationWorker.perform_bulk(*migrations)
     end
   end
 

spec/migrations/migrate_stage_id_reference_in_background_spec.rb

@@ -8,6 +8,8 @@ describe MigrateStageIdReferenceInBackground, :migration, :redis do
   let(:projects) { table(:projects) }
 
   before do
+    stub_const('MigrateStageIdReferenceInBackground::BATCH_SIZE', 1)
+
     projects.create!(id: 123, name: 'gitlab1', path: 'gitlab1')
     pipelines.create!(id: 1, project_id: 123, ref: 'master', sha: 'adf43c3a')