In trying to undesrtand why such a negative reaction, soundcloud's take on this goes beyond just ruby or rails.

They detail an approach that goes extends tzs' suggestion:

1. Get the maximum primary key value for the table

2. Create new table and journal table

3. Activate journalling with triggers

4. Perform alter statement on new table

5. Copy in chunks up to max primary key value to new table

6. Switch new and original table names and remove triggers

7. Replay journal: insert, update, deletes

Not only did the have success with this approach, they studied Facebook's approach[1], and Twitter's[2], and explain why it didn't work for them.

[1] https://github.com/freels/table_migrator

[2] http://www.facebook.com/note.php?note_id=430801045932

Presumably not that exact algorithm.

There's a race between (1) and (3) where new entries could be created (increasing the max primary key value) before the triggers are in place.