Migrating from a Legacy Datasource

Refactoring a project in Rails can be a handful, especially when the original database does not conform to Rails standards. Lately, I've found it much quicker to simply redesign the data model, and migrate the data from the old database.

I've had to tackle this problem a couple of times now, and feel like I've refined the process a great deal. I don't know if this is the correct way of doing things, I'm just saying, it works for me.

Step #1 - Map the legacy database

1. Create a new file under config/initializers called legacy.rb
2. Create a new module, we'll call it "Legacy"
   module Legacy
end
3. Map your original database as best you can to rails models.
   module Legacy

    class ActiveRecordLegacy < ActiveRecord::Base
        self.establish_connection(
            :adapter => 'mysql',
            :host => 'localhost',
            :username => 'username',
            :password => 'password',
            :database => 'legacy_database'
        )
    end

    class LegacyTable < ActiveRecordLegacy
        set_table_name :legacy_table
        set_primary_key :LegacyTableID
    end

end

Step #2 - Create your migration script

1. Create a new file model under app/models called migrate.rb
   class Migrate
end
2. Fill in your migration steps!
   class Migrate
    def self.run
        Migrate.migrate_legacy_table()
    end

    def self.migrate_legacy_table
        Legacy::LegacyTable.find(:all).each { |record|
             # Do some nifty migrations here
        }
    end
end
3. Load up the rails console: ruby ./script/console
   >> Migrate.run


Again, I don't know if this is the "correct" method, it's just how I do things.

-Butch