Do not schedule bg migration when it is not needed

db/post_migrate/20170628080858_migrate_stage_id_reference_in_background.rb

@@ -17,7 +17,7 @@ class MigrateStageIdReferenceInBackground < ActiveRecord::Migration
   # It will take around 3 days to process 20M ci_builds.
   #
   def up
-    Build.all.each_batch(of: BATCH_SIZE) do |relation, index|
+    Build.where(stage_id: nil).each_batch(of: BATCH_SIZE) do |relation, index|
       relation.each_batch(of: RANGE_SIZE) do |relation|
         range = relation.pluck('MIN(id)', 'MAX(id)').first
 

spec/migrations/migrate_stage_id_reference_in_background_spec.rb

@@ -51,8 +51,7 @@ describe MigrateStageIdReferenceInBackground, :migration, :sidekiq do
         expect(described_class::MIGRATION).to be_scheduled_migration(2.minutes, 1, 2)
         expect(described_class::MIGRATION).to be_scheduled_migration(2.minutes, 3, 3)
         expect(described_class::MIGRATION).to be_scheduled_migration(4.minutes, 4, 5)
-        expect(described_class::MIGRATION).to be_scheduled_migration(4.minutes, 6, 6)
-        expect(BackgroundMigrationWorker.jobs.size).to eq 4
+        expect(BackgroundMigrationWorker.jobs.size).to eq 3
       end
     end
   end