Replacing ActiveRecord
with DataMapper
in Ruby on Rails
Peter Degen-Portnoy
November 25, 2008

DataMapper is an ORM

A lot like ActiveRecord
●
DB Adapters
●
Migrations
●
Associations
●
One to one
●
One to many
●
Many to many
●
Many to one

Application Rails Merb
ORM DataMapper
Adaptors DO.rb YAML IMAP
Storage

Most often used with Merb
...but not exclusively

DataMapper Goodness
●
Identity Map
●
Mappings in Model
●
Multiple Repositories
●
Lazy Loads of Large Lumps
●
Strategic Eager Loading

Identity Map
●
One object for one row
●
Reduces hits to database
●
Exists during Identity Map Session

Mapping in Model
●
Persist any class
●
Independent data store evolution
●
Connect to multiple databases
●
DM issues updates or creates only for what
it knows

Multiple Repositories
●
Nested YAML structure
●
Override target table names
●
Specify connection per class or even call

Lazy Loading
●
Reduces load of large columns
●
Text
●
Blobs
●
Graphics
●
Specify which columns lazy load
●
Lazy loads happen together; only 1 more call
●
Can be grouped

Strategic Eager Loading
●
One call per child
●
Eliminates n+1 calls for associations

Way faster than ActiveRecord

10,000 records fetched w/ Active Record
Average: 1.3429 seconds

10,000 records fetched w/ DataMapper
Average: 0.0002 seconds

1.3429 seconds compared to 0.0002 seconds

0.0150% as much time

To Fetch 1,000 Records
ActiveRecord Over 204,000 calls
DataMapper Under 7,000 calls

Replacing ActiveRecord with DataMapper
the part for which you have been waiting

Demo Applications
●
One application to test ActiveRecord
●
One application to test DataMapper
●
Each create same data structure:
●
script/generate model book amount:float
date:date name:string description:text
--skip-timestamps
●
Create 1000 records
●
Profile & Benchmark

Gems You Need
●
data_mapper
●
do_mysql (or do_postgres or do_sqlite3)
●
dm-core
●
fastthread
●
json
●
rspec

Edit config/environment.rb
●
config.frameworks -= [ :active_record ]
●
config.gem “do_mysql”
●
config.gem “dm-core”
●
config.gem “dm-migrations”
Make sure your gems are up to date by
running “sudo rake gems:install”

Configure database.yml
development: &defaults
:adapter: mysql
:database: dm2_dev
:user: root
:password: mysql
:socket: /var/run/mysqld/mysqld.sock
:host: localhost
test:
<<: *default
:database: dm2_test
production:
<<: *default
:database: dm2_prod

Create
config/initializers/datamapper.rb
require \"dm-core\"
hash = YAML.load(File.new(Rails.root + \"/config/database.yml\"))
DataMapper.setup(:default, 'mysql://root:mysql@localhost/dm2_dev' )

Generate Model
> script/generate dm_model book amount:float date:date name:string
description:text --skip-migration
NOTE: --skip-migration is needed because the rails-datamapper integration
gem doesn't pull out the ActiveRecord dependency on migration creation
yet.

Finish model definition
require 'lorem'
class Book
include DataMapper::Resource
property :id, Integer, :serial => true
property :amount, Float
property :date, Date
property :name, String
property :description, Text, :lazy => true
def self.create_dummy_data(num)
num.times do |number|
t = Book.new
t.attributes = {:amount => number, :date => Time.now(),
:name => Lorem.lorem(4, false), :description => Lorem.lorem(60, false)}
t.save
end
end
end

Create Migration
migration 1, :create_books do
up do
create_table :books do
column :id, Integer, :serial => true, :nullable? => false, :key => true
column :amount, Float
column :date, Date
column :name, String
column :description, DataMapper::Types::Text
end
end
down do
drop_table :books
end
end

Create Migration Rake Task
namespace :dm do
task :migrate => :environment do
gem \"dm-migrations\"
require \"migration_runner\"
Dir[File.join(Rails.root, \"db\", \"migrate\", \"*\")].each {|f| require f}
migrate_up!
end
end
src/dm2> mysqladmin create dm2_dev
src/dm2> rake dm:migrate

Wire up application
●
Create Controller for book
●
Create “bulk” generation capability

Summary Checklist
●
Add gems & generate application
●
Remove ActiveRecord & add DataMapper
●
Configure database.yml & datamapper.rb
●
Create your database
●
Generate your model, complete definition &
migrate
●
Continue defining application

Additional Goodness
●
Validations
●
Conditions
●
Order
●
Aggregations
●
Associations
●
Named Scope (class methods)

Assessment
●
Can't beat the speed
●
Outstanding flexibility
●
Durn if it won't be more complex to develop
with

Thank you