discourse/db/migrate/20121123054127_make_post_number_distinct.rb

class MakePostNumberDistinct < ActiveRecord::Migration
  def up

    Topic.exec_sql('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')

    remove_index :posts, :forum_thread_id_and_post_number
    add_index :posts, [:topic_id, :post_number], :unique => true
  end

  def down
    # don't want to mess with the index ... its annoying 
    raise ActiveRecord::IrreversibleMigration
  end
end