Programming Tutorials

Active Record in Ruby on Rails

By: David Heinemeier Hansson in Ruby Tutorials on 2009-03-03  

Active Record connects business objects and database tables to create a persistable domain model where logic and data are presented in one wrapping. It's an implementation of the object-relational mapping (ORM) pattern by the same name as described by Martin Fowler:

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

Active Record's main contribution to the pattern is to relieve the original of two stunting problems: lack of associations and inheritance. By adding a simple domain language-like set of macros to describe the former and integrating the Single Table Inheritance pattern for the latter, Active Record narrows the gap of functionality between the data mapper and active record approach.

A short rundown of the major features:

• Automated mapping between classes and tables, attributes and columns.

   class Product < ActiveRecord::Base; end
  
   ...is automatically mapped to the table named "products", such as:
  
   CREATE TABLE products (
     id int(11) NOT NULL auto_increment,
     name varchar(255),
     PRIMARY KEY  (id)
   );
  
   ...which again gives Product#name and Product#name=(new_name)

• Associations between objects controlled by simple meta-programming macros.

   class Firm < ActiveRecord::Base
     has_many   :clients
     has_one    :account
     belongs_to :conglomorate
   end

• Aggregations of value objects controlled by simple meta-programming macros.

   class Account < ActiveRecord::Base
     composed_of :balance, :class_name => "Money",
                 :mapping => %w(balance amount)
     composed_of :address,
                 :mapping => [%w(address_street street), %w(address_city city)]
   end

• Validation rules that can differ for new or existing objects.

    class Account < ActiveRecord::Base
      validates_presence_of     :subdomain, :name, :email_address, :password
      validates_uniqueness_of   :subdomain
      validates_acceptance_of   :terms_of_service, :on => :create
      validates_confirmation_of :password, :email_address, :on => :create
    end

• Callbacks as methods or queues on the entire lifecycle (instantiation, saving, destroying, validating, etc).

   class Person < ActiveRecord::Base
     def before_destroy # is called just before Person#destroy
       CreditCard.find(credit_card_id).destroy
     end
   end
  
   class Account < ActiveRecord::Base
     after_find :eager_load, 'self.class.announce(#{id})'
   end

• Observers for the entire lifecycle

   class CommentObserver < ActiveRecord::Observer
     def after_create(comment) # is called just after Comment#save
       Notifications.deliver_new_comment("[email protected]", comment)
     end
   end

• Inheritance hierarchies

   class Company < ActiveRecord::Base; end
   class Firm < Company; end
   class Client < Company; end
   class PriorityClient < Client; end

• Transactions

    # Database transaction
    Account.transaction do
      david.withdrawal(100)
      mary.deposit(100)
    end

• Reflections on columns, associations, and aggregations

    reflection = Firm.reflect_on_association(:clients)
    reflection.klass # => Client (class)
    Firm.columns # Returns an array of column descriptors for the firms table

• Direct manipulation (instead of service invocation):

     long pkId = 1234;
     DomesticCat pk = (DomesticCat) sess.load( Cat.class, new Long(pkId) );
     // something interesting involving a cat...
     sess.save(cat);
     sess.flush(); // force the SQL INSERT
  

  Active Record lets you:

     pkId = 1234
     cat = Cat.find(pkId)
     # something even more interesting involving the same cat...
     cat.save

• Database abstraction through simple adapters (~100 lines) with a shared connector

   ActiveRecord::Base.establish_connection(:adapter => "sqlite", :database => "dbfile")
  
   ActiveRecord::Base.establish_connection(
     :adapter  => "mysql",
     :host     => "localhost",
     :username => "me",
     :password => "secret",
     :database => "activerecord"
   )

• Logging support for Log4r and Logger

    ActiveRecord::Base.logger = Logger.new(STDOUT)
    ActiveRecord::Base.logger = Log4r::Logger.new("Application Log")

• Database agnostic schema management with Migrations

    class AddSystemSettings < ActiveRecord::Migration
      def self.up
        create_table :system_settings do |t|
          t.string :name
          t.string :label
          t.text :value
          t.string :type
          t.integer  :position
        end
  
        SystemSetting.create :name => "notice", :label => "Use notice?", :value => 1
      end
  
      def self.down
        drop_table :system_settings
      end
    end