Add stage_id column to ci_builds in a separate migration

db/migrate/20170526185602_add_stage_id_to_ci_builds.rb

+class AddStageIdToCiBuilds < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    add_column :ci_builds, :stage_id, :integer
+  end
+end

db/post_migrate/20170526101042_migrate_pipeline_stages.rb

@@ -16,8 +16,6 @@ class MigratePipelineStages < ActiveRecord::Migration
           ORDER BY stage_idx
     SQL
 
-    add_column(:ci_builds, :stage_id, :integer)
-
     stage_id = Arel.sql('(SELECT id FROM ci_stages ' \
                          'WHERE ci_stages.pipeline_id = ci_builds.commit_id ' \
                          'AND ci_stages.name = ci_builds.stage)')