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The migration that cannot be undone: Irreversible Migration

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source link: https://www.devroom.io/2008/05/06/the-migration-that-cannot-be-undone-irreversible-migration/?utm_campaign=Feed%3A+ariejan+%28ariejan%7Cdevroom.io%29
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The migration that cannot be undone: Irreversible Migration

Posted: 2008-05-06 - Last updated: 2019-06-05

Tagged 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:

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


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