Rails and Alternative
ORMs
Dan Weinand
Jonathan Dahl

Why?

DB vs. OO
declarative vs. imperative

DB vs. OO
char/text vs. string

DB vs. OO
schema vs. _______

DB vs. OO
joins vs. pointers

DB vs. OO
public vs. encapsulated

DB vs. OO
constraints vs. validation

DB vs. OO
_____ vs. inheritance

DB vs. OO
DBA vs. programmer

DB vs. OO
structure vs. behavior

http://www.engrish.com//wp-content/uploads/2008/08/hand-grenade.jpg

1. ActiveRecord

An object that wraps a row in a database table or view,
encapsulates the database access, and adds domain logic on
that data.
Martin Fowler, PoEAA, 160.

One object per row.

Data access and domain
logic are done together.

6 Criticisms
of ActiveRecord

1. Too much magic.
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class User < ActiveRecord::Base
end

2. Not performant.
http://www.flickr.com/photos/danprates/1418154518/

3. Not thread safe.
No connection pooling.
http://www.flickr.com/photos/streetmuseo/57990915/

4. Hides SQL from you.
http://www.momentaryfascinations.com/entertainment/the.lost.monty.python.cartoon/still.5.jpg

5. Hard to fight against.
http://www.mmonotebook.com/wp-content/uploads/2008/04/larp.jpg

6. Encourages stupid databases.

9 ways to bypass
application-level
constraints

1. Fixtures

2. Klass.delete(1)

3. record.save(false)

4.

5. Share DB with a
second app

6. User.create(:company_id => 1)

class User < ActiveRecord::Base
7. has_many :posts
end

8. ActiveRecord::Base.connection.execute()

9. Errors in your app
logic

2. Sequel

An object that wraps a row in a database table or view,
encapsulates the database access, and adds domain logic on
that data.
Martin Fowler, PoEAA, 160.

Ruby DSL for SQL
posts = DB[:posts]
posts.filter(:category => /ruby/i, :author => [:jon, :dan])
posts.reverse_order(:created_at, :updated_at)
posts.select(:title, :author)
posts.limit(10)

Ruby-like DSL for SQL
posts.filter(:category => :ruby).order(:created_at).select(:title, :author)

ORM
class Post < Sequel::Model
many_to_many :tags
many_to_one :author
validates_presence_of :title
end
post = Post[123]
post.title # \"rails orms\"
post.title = \"Rails ORMs\"
post.save

3 interesting things
sequel postgres://localhost:5432/myapp_development

3 interesting things
require 'memcache'
CACHE = MemCache.new 'localhost:11211', :namespace => 'blog'
class Author < Sequel::Model
set_cache CACHE, :ttl => 3600
end
Author[333] # database hit
Author[333] # cache hit

3 interesting things
DB = Sequel.sqlite('app_test.db',
:servers=>{:read_only=>{:host=>'slave_server'}})

3. Datamapper

A layer of Mappers (473) that moves data between objects and
a database while keeping them independent of each other and
the mapper itself.
Martin Fowler, PoEAA, 165.

Multiple Databases
• SQL-based
• In-memory
• Document-based
• Search engines

Explicit property mapping
class Message
include DataMapper::Resource
property :name, String
property :body, Text
property :created_at, DateTime
validates_presence_of :name
end

Get only the data you need

Minimize the number of
queries

Save only the data that
has changed

Identity Map
repository do
@parent.children.each do |child|
puts @parent.object_id = child.parent.object_id
end
end

Ruby Query Syntax
Zoo.first(:name.eql => 'Galveston')
Person.all(:age.gt => 30)
Person.all(:age.gte => 30)
Person.all(:name.not => 'bob')
Person.all(:name.like => 'S%', :id => [1,2,3,4,5])
Person.all(:name.not => ['bob','rick','steve'])
Person.all(:order => [:age.desc])

Chainable finders
class Zoo
def self.open
all(:open => true)
end
def self.big
all(:animal_count.gte => 1000)
end
end
big_open_zoos = Zoo.big.open

Associations
class Article
include DataMapper::Resource
property :id, Serial
property :title, String
property :content, Text
belongs_to :author
end
class Author
include DataMapper::Resource
property :id, Serial
property :name, String
has n, :articles
end
Article.all('author.name' => 'Michael')

Implicit and explicit
migrations

Only basic functionality
in core

Gems for everything else

Is it ready for
primetime?

Roadmap to 1.0

Rails integration still
lacking

Not many production
deployments

Documentation sucks

Some plugins still not
ported

Thanks!
Dan Weinand
Jon Dahl