Agile Web Development Using Rails 6/

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When Migrations Go Bad

When Migrations Go Bad

Learn about the downside of migrations.

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Overview

Migrations suffer from one serious problem. The underlying DDL statements that update the database schema are not transactional. This isn’t exclusively a failing in Rails, though, as most databases don’t support the rolling back of create table, alter table, and other DDL statements.

Let’s look at a migration that tries to add two tables to a database:

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class ExampleMigration < ActiveRecord::Migration 
  def change
    create_table :one do ... 
    end
    create_table :two do ... 
    end
  end 
end

In the normal course of events, the up() method adds tables one and two, and the down() method removes them.

What happens if ...