Add initial build stage_id ref background migration

db/post_migrate/20170628080858_migrate_stage_id_reference_in_background.rb

@@ -3,7 +3,17 @@ class MigrateStageIdReferenceInBackground < ActiveRecord::Migration
 
   DOWNTIME = false
 
+  disable_ddl_transaction!
+
+  class Build < ActiveRecord::Base
+    self.table_name = 'ci_builds'
+  end
+
   def up
+    Build.find_each do |build|
+      BackgroundMigrationWorker
+        .perform_async('MigrateBuildStageIdReference', [build.id])
+    end
   end
 
   def down

lib/gitlab/background_migration/migrate_build_stage_id_reference.rb

@@ -1,15 +1,19 @@
 module Gitlab
   module BackgroundMigration
     class MigrateBuildStageIdReference
-      class Build < ActiveRecord::Base
-        self.table_name = 'ci_builds'
-      end
-
-      class Stage < ActiveRecord::Base
-        self.table_name = 'ci_stages'
-      end
-
       def perform(id)
+        raise ArgumentError unless id.is_a?(Integer)
+
+        sql = <<-SQL.strip_heredoc
+          UPDATE "ci_builds" SET "stage_id" = (
+            SELECT id FROM ci_stages
+              WHERE ci_stages.pipeline_id = ci_builds.commit_id
+              AND ci_stages.name = ci_builds.stage
+            )
+          WHERE "ci_builds"."id" = #{id} AND "ci_builds"."stage_id" IS NULL
+        SQL
+
+        ActiveRecord::Base.connection.execute(sql)
       end
     end
   end

spec/migrations/migrate_stage_id_reference_in_background_spec.rb

@@ -22,5 +22,10 @@ describe MigrateStageIdReferenceInBackground, :migration, :redis do
   end
 
   it 'schedules background migrations' do
+    expect(jobs.where(stage_id: nil)).to be_present
+
+    migrate!
+
+    expect(jobs.where(stage_id: nil)).to be_empty
   end
 end