Schedule background migration only when it is needed

db/post_migrate/20170628080858_migrate_stage_id_reference_in_background.rb

@@ -12,7 +12,9 @@ class MigrateStageIdReferenceInBackground < ActiveRecord::Migration
   end
 
   def up
-    Build.find_in_batches(batch_size: BATCH_SIZE).with_index do |builds, batch|
+    Build.where(stage_id: nil)
+      .find_in_batches(batch_size: BATCH_SIZE)
+      .with_index do |builds, batch|
         migrations = builds.map { |build| [MIGRATION, [build.id]] }
 
         BackgroundMigrationWorker.perform_bulk(*migrations)