The migration that cannot be undone: Irreversible Migration
6 May, 2008

Migrations have up and down methods, as we all know. But in some cases, your up method does things you can’t undo in your down method.

For example:

def self.up
# Change the zipcode from the current :integer to a :string type.
end

Now, converting integers to strings will always work. But, you feel it coming, converting a string into an integer will not always be possible. In other words, we can’t reverse this migration.

That’s why we should raise ActiveRecord::IrreversibleMigration in the down method.

def self.down
raise ActiveRecord::IrreversibleMigration
end

Now, if you run your migration (upwards), you’ll see it being applied like it shoud. However, if you try to go back, you’ll see rake aborting with ActiveRecord::IrreversibleMigration.

$rake db:migrate VERSION=4 -- Database is migrated$ rake db:migrate VERSION=3
-- Rake aborted!
-- ActiveRecord::IrreversibleMigration

So, if you have things you can’t undo, raise ActiveRecord::IrreversibleMigration in your migration’s down method.