41 lines
1.2 KiB
Markdown
41 lines
1.2 KiB
Markdown
+++
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date = "2008-05-06"
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title = "The migration that cannot be undone: Irreversible Migration"
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tags = ["General"]
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slug = "the-migration-that-cannot-be-undone-irreversible-migration"
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+++
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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.
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For example:
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```ruby
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def self.up
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# Change the zipcode from the current :integer to a :string type.
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change_column :address, :zipcode, :string
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end
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```
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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.
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That's why we should raise ActiveRecord::IrreversibleMigration in the down method.
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```ruby
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def self.down
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raise ActiveRecord::IrreversibleMigration
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end
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```
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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.
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```sh
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$ rake db:migrate VERSION=4
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-- Database is migrated
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$ rake db:migrate VERSION=3
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-- Rake aborted!
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-- ActiveRecord::IrreversibleMigration
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```
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So, if you have things you can't undo, raise ActiveRecord::IrreversibleMigration in your migration's down method.
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