class MakePostNumberDistinct < ActiveRecord::Migration[4.2] def up DB.exec('update posts p set post_number = calc from ( select id, post_number, topic_id, row_number() over (partition by topic_id order by post_number, created_at) calc from posts where topic_id in ( select topic_id from posts group by topic_id, post_number having count(*)>1 ) ) as X where calc <> p.post_number and X.id = p.id') end def down # don't want to mess with the index ... its annoying raise ActiveRecord::IrreversibleMigration end end