Question

I made a mistake in editing a rails migration (I’m using rails 3.1.0rc5). So I corrected it and attempted to do a rake db:rollback followed by a rake db:migrate db:test:prepare. But the rollback failed for some reason. Here’s the rollback portion of the migration:

  def down
    drop_table :assets
  end

this is the response from db:rollback:

==  CreateAssets: reverting ===================================================
rake aborted!
An error has occurred, this and all later migrations canceled:

ActiveRecord::IrreversibleMigration

Tasks: TOP => db:rollback
(See full trace by running task with --trace)

so I just manually deleted the assets table in the database (I’m just using the sqlite3 dbs), commented out the drop_table :assets line and ran db:rollback again:

==  CreateAssets: reverting ===================================================
-- drop_table("assets")
rake aborted!
An error has occurred, this and all later migrations canceled:

SQLite3::SQLException: no such table: assets: DROP TABLE "assets"

Tasks: TOP => db:rollback
(See full trace by running task with --trace)

Why is it trying to drop the assets table? I commented out (and actually later deleted) that drop_table :assets line from the migration. I even did a grep -r assets to see where rails might be getting that drop command from, but couldn’t find anything.

So my question is how do I get this migration re-run? And I’m curious where rails is getting that drop command?

UPDATE: It turns out that rails 3.1 actually doesn’t look at the migration class’ down method when rolling back! It looks at the change method’s migration commands and reverses them. That’s why rake db:rollback was trying to drop the assets table even after I removed the drop_table :assets instruction! How I fixed this is I changed the change method to the old rails 3.0 style up method. Then rails looked at the down method which I had removed the drop_table command from and the rollback was successful.

Answer 1

Migrations changed a fair bit with the latest rails, especially reversing migrations.
I think http://shardul.wordpress.com/2012/01/25/reversible-migrations/ will provide you with the info and options you need.

btw I had a lot of issues with the 3.1. rc candidates (e.g. rc4, rc5) and have had to stay at 3.0.9 for stable new apps right now.