---
title: "The migration that cannot be undone: Irreversible Migration"
kind: article
slug: the-migration-that-cannot-be-undone-irreversible-migration
created_at: 2008-05-06
tags:
  - General
---

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:

<pre lang="ruby">def self.up
  # Change the zipcode from the current :integer to a :string type.
  change_column :address, :zipcode, :string
end</pre>

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.

<pre lang="ruby">def self.down
  raise ActiveRecord::IrreversibleMigration
end</pre>

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.

<pre lang="sh">$ rake db:migrate VERSION=4
-- Database is migrated
$ rake db:migrate VERSION=3
-- Rake aborted!
-- ActiveRecord::IrreversibleMigration</pre>

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