Agile Web Development with Rails 7.2

When Migrations Go Bad

Migrations suffer from one serious problem. The underlying DDL statements that update the database schema are not transactional. This isn’t a failing in Rails—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:

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.

But what happens if there’s a problem creating the second table? We’ll end up with a database containing table one but not ...