Add initial stage_id background migration files

db/post_migrate/20170628080858_migrate_stage_id_reference_in_background.rb

+class MigrateStageIdReferenceInBackground < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+  end
+
+  def down
+    # noop
+  end
+end

lib/gitlab/background_migration/migrate_build_stage_id_reference.rb

+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)
+      end
+    end
+  end
+end

spec/migrations/migrate_stage_id_reference_in_background_spec.rb

+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20170628080858_migrate_stage_id_reference_in_background')
+
+describe MigrateStageIdReferenceInBackground, :migration, :redis do
+  let(:jobs) { table(:ci_builds) }
+  let(:stages) { table(:ci_stages) }
+  let(:pipelines) { table(:ci_pipelines) }
+  let(:projects) { table(:projects) }
+
+  before do
+    projects.create!(id: 123, name: 'gitlab1', path: 'gitlab1')
+    pipelines.create!(id: 1, project_id: 123, ref: 'master', sha: 'adf43c3a')
+
+    jobs.create!(id: 1, commit_id: 1, project_id: 123, stage_idx: 2, stage: 'build')
+    jobs.create!(id: 2, commit_id: 1, project_id: 123, stage_idx: 2, stage: 'build')
+    jobs.create!(id: 3, commit_id: 1, project_id: 123, stage_idx: 1, stage: 'test')
+    jobs.create!(id: 4, commit_id: 1, project_id: 123, stage_idx: 3, stage: 'deploy')
+
+    stages.create(id: 101, pipeline_id: 1, project_id: 123, name: 'test')
+    stages.create(id: 102, pipeline_id: 1, project_id: 123, name: 'build')
+    stages.create(id: 103, pipeline_id: 1, project_id: 123, name: 'deploy')
+  end
+
+  it 'schedules background migrations' do
+  end
+end