Use Trucker to migrate legacy data into your Rails app.

1. Install the trucker gem

sudo gem install trucker

2. Add trucker to your config.gem block in environment.rb.

config.gem "trucker"

3. Generate the basic trucker files

script/generate truck

This will do the following things:

• Add legacy adapter to database.yml
• Add app/models/legacy directory
• Add app/models/legacy to autoload_paths in Rails Initializer config block
• Add app/models/legacy/legacy_base.rb (from which legacy models will inherit)
• Add legacy sub classes for all existing models
• Generate sample migration task (using pluralized model names)

4. Update the legacy database adapter in database.yml with your legacy database info

legacy:
  adapter: mysql
  encoding: utf8
  database: app_legacy
  username: root
  password:

By convention, we recommend naming your legacy database APP_legacy, just as your other databases might be named APP_development, APP_production, etc.

5. If the legacy database doesn't already exist, add it.

rake db:create:all

6. Import your legacy data into the legacy database.

mysql -u root app_legacy < old_database.sql

If you're not using mysql, you should change this command as needed.

7. Custom your table name for each of your legacy models.

class LegacyPost < LegacyBase
  self.table_name =  "LEGACY_TABLE_NAME_GOES_HERE"
end

Since you're migrating data from an old database, your table names may not follow Rails conventions for database table naming. If so, you will need to set the self.table_name = value for each of your legacy models to match the name of table from which you will be importing data.

For instance, in the example above, if your old posts were stored in an articles table, you would customize self.table_name = like so:

class LegacyPost < LegacyBase
  self.table_name =  "articles"
end

8. Update legacy model field mappings.

class LegacyPost < LegacyBase
  self.table_name =  "LEGACY_TABLE_NAME_GOES_HERE"

  def map
    {
      :headline => self.title.squish,
      :body => self.long_text.squish
    }
  end
end

This is where you will connect your old database attributes with your new ones. The map method is really just a hash which uses your new model attribute names as keys and your legacy model attributes as values.

(aka :new_field => self.legacy_field)

Note: make sure to add self. to each legacy attribute name.

9. Need to tweak some data? Just add some core ruby methods or add a helper method.

class LegacyPost < LegacyBase
  self.table_name =  "LEGACY_TABLE_NAME_GOES_HERE"

  def map
    {
      :headline => self.title.squish.capitalize, # <= Added capitalize method
      :body => tweak_body(self.long_text.squish) # <= Added tweak_body method
    }
  end

  # Insert helper methods as needed
  def tweak_body(body)
    body = body.gsub(/<br \//,"\n") # <= Convert break tags into normal line breaks
    body = body.gsub(/teh/, "the")  # <= Fix common typos
  end
end

10. Start migrating!

rake db:migrate:posts

Trucker supports a few command line options when migrating records:

rake db:migrate:posts limit=100 (migrates 100 records)
rake db:migrate:posts limit=100 offset=100 (migrates 100 records, but skip the first 100 records)

You can tweak the default migration output generated by Trucker by using the :label option.

rake db:migrate:posts
=> Migrating posts

rake db:migrate:posts, :label => "blog posts"
=> Migrating blog posts

Trucker works great for migrating data from many legacy data sources such as apps built with PHP, Perl, Python, or even older versions of Rails (where upgrading an existing Rails code base is not practical). But, if you're migrating data from a large enterprise system, Trucker may not be your best choice.

That said, if you need to pull off a complex migration for a model, you can use a custom helper method to override Trucker's default migrate method in your rake task.

namespace :db do
  namespace :migrate do
    ...
    desc 'Migrate pain_in_the_ass model'
    task :pain_in_the_ass => :environment do
      Trucker.migrate :pain_in_the_ass, :helper => pain_in_the_ass_migration
    end 
  end
end

def pain_in_the_ass_migration
  # Custom code goes here
end

If you don't want to write your custom migration method from scratch, you can copy trucker's migrate method method from lib/trucker.rb and tweak accordingly.

As an example, here's a custom helper used to migrate join tables on a bunch of models.

namespace :db do
  namespace :migrate do

    desc 'Migrates join tables'
    task :joins => :environment do
      migrate :joins, :helper => :migrate_joins  
    end

  end
end

def migrate_joins
  puts "Migrating #{number_of_records || "all"} joins #{"after #{offset_for_records}" if offset_for_records}"

  ["chain", "firm", "function", "style", "website"].each do |model|

    # Start migration
    puts "Migrating theaters_#{model.pluralize}"

    # Delete existing joins
    ActiveRecord::Base.connection.execute("TRUNCATE table theaters_#{model.pluralize}")

    # Tweak model ids and foreign keys to match model syntax
    if model == 'website'
      model_id = "url_id"
      send_foreign_key = "url_id".to_sym
    else
      model_id = "#{model}_id"
      send_foreign_key = "#{model}_id".to_sym
    end

    # Create join object class
    join = Object.const_set("Theaters#{model.classify}", Class.new(ActiveRecord::Base))

    # Set model foreign key
    model_foreign_key = "#{model}_id".to_sym

    # Migrate join (unless duplicate)
    "LegacyTheater#{model.classify}".constantize.find(:all, with(:order => model_id)).each do |record|
  
      unless join.find(:first, :conditions => {:theater_id => record.theater_id, model_foreign_key => record.send(send_foreign_key)})
        attributes = {
          model_foreign_key => record.send(send_foreign_key),
          :theater_id => record.theater_id
        }
    
        # Check if theater chain is current
        attributes[:is_current] = {'Yes' => 1, 'No' => 0, '' => 0}[record.current] if model == 'chain'
    
        # Migrate join
        join.create(attributes)
      end
    end
  end
end

Check out the Trucker sample app for a working example of Trucker-based legacy data migration.

Trucker is based on a migration technique using legacy models first pioneered by Dave Thomas: http://pragdave.blogs.pragprog.com/pragdave/2006/01/sharing_externa.html

• Fork the project.
• Make your feature addition or bug fix.
• Add tests for it. This is important so we don't break a future version unintentionally.
• Commit your changes, but do not mess with the rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself so we can ignore when we pull)
• Send a pull request. Bonus points for topic branches.

• Patrick Crowley
• Rob Kaufman
• Jordan Fowler
• Roel Bondoc
• Olivier Lacan

Copyright (c) 2014 Patrick Crowley and Rob Kaufman. See LICENSE for details.