Migrating Legacy Data (Ruby Midwest)

• Develop new site
• Migrate data
• Deploy new site

• Dump database
• Drop some tables
• Rename attributes
• Tweak data by hand
• Import data into new app
• Pray things are okay

• Labor intensive
• Error prone
• One way trip

Migrate legacy data
(with less suck)

• Migrate data from day one
• Improve data over time
• Launch with conﬁdence

rake +
legacy classes +
migration helper

$ script/generate truck
create app/models/legacy
create app/models/legacy/legacy_base.rb
exists lib/tasks
create lib/tasks/legacy.rake
insert added legacy adapter to end of database.yml
insert added new load path to environment.rb

conﬁg/environment.rb
Rails::Initializer.run do |config|
config.load_paths += %W( #{RAILS_ROOT}/app/
models/legacy )
end

conﬁg/database.yml
legacy:
adapter: mysql
encoding: utf8
database: trucker_legacy
username: root
password:

app/models/legacy/legacy_base.rb
class LegacyBase < ActiveRecord::Base
self.abstract_class = true
establish_connection "legacy"

def migrate
new_record = self.class.to_s.gsub(/
Legacy/,'::').constantize.new(map)
new_record[:id] = self.id
new_record.save
end

app/models/legacy/legacy_post.rb
class LegacyPost < LegacyBase
set_table_name "blog_posts"

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

end

app/models/legacy/legacy_post.rb
class LegacyPost < LegacyBase
set_table_name "blog_posts"

def map
{
:name => tweak(self.headline.squish),
:body => self.body.squish
}
end

def tweak(name)
name.capitalize.gsub(/teh/, "the")
end

lib/tasks/legacy.rake
namespace :db do
namespace :migrate do

desc 'Migrates posts'
task :posts => :environment do
Trucker.migrate :posts
end

end
end

Don’t forget to import
your legacy database!

$ rake db:migrate:posts

Migrating all posts (1/10)

$ rake db:migrate:posts limit=5

Migrating 5 posts (1/10)

$ rake db:migrate:posts limit=5 offset=5

Migrating 5 posts after 5 (6/10)

Use helper method for
custom migrations

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

But you still need to
pack your stuff up.

Some other things
to think about

• MySQL issues:
“ALTER TABLE mytable
CONVERT TO CHARACTER
SET utf8;

• iconv

JDBC Adapters vs
ActiveRecord

• There are about 12 adapters in
the wild for pure ActiveRecord

• The ActiveRecord adapter for
sqlserver runs on Win/Unix,
but Unix setup is complicated

• Text adapter does CSV, Tab,
and other plain text formats

Migrating Legacy Data (Ruby Midwest)

ActiveRecord Tested:
 MySQL  Derby
 PostgreSQL  HSQLDB

 Oracle  H2

 Microsoft SQL  SQLite3

Server  Informix

 DB2

 FireBird

• Download MSSQL driver
(Google for it, they move it
around a lot)

• Copy sqljdbc4.jar into
RAILS_ROOT/lib

• Add require 'lib/sqljdbc4.jar' at
the top of your environment.rb
or application.rb

conﬁg/database.yml
legacy:
adapter: jdbc
username: USERNAME
password: PASSWORD
driver: com.microsoft.sqlserver.jdbc.SQLServerDriver
jdbc: sqlserver://63.134.199.59:1433

• Run in prod when you can
• Run locally when you can't
(Heroku, VPNs, etc)

• http://github.com/mokolabs/trucker

• http://github.com/mokolabs/
trucker_sample_app

• http://spkr8.com/t/3749

Migrating Legacy Data (Ruby Midwest)

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