The migration that cannot be undone: Irreversible Migration

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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2 Responses to “The migration that cannot be undone: Irreversible Migration”

  1. billitch says:

    thanks !is there a way to force the irreversible migration from rake, or allow it only in development code ?

  2. @billitch: no, you cannot force “the way back”. Of course, you can comment out the exception, but when you ‘re-migrate’ you’ll be in trouble, because you are doing the same migration twice.

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