File: README

# README

Path: | README
Last Update: | Mon Aug 18 21:27:05 -0600 2008

# Active Record — Object-relation mapping put on rails

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)

  Learn more

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

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

  Learn more

- 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

  Learn more

- 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

  Learn more

- 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

  Learn more

- Observers for the entire lifecycle

       class CommentObserver < ActiveRecord::Observer
         def after_create(comment) # is called just after Comment#save
           Notifications.deliver_new_comment("david@loudthinking.com", comment)
         end
       end

  Learn more

- Inheritance hierarchies

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

  Learn more

- Transactions

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

  Learn more

- 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

  Learn more

- Direct manipulation (instead of service invocation)

  So instead of (Hibernate example):

         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

  Learn more

- 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"
       )

  Learn more and read about the built-in support for MySQL, PostgreSQL, SQLite, Oracle, SQLServer, and DB2.

- 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

  Learn more

## Simple example (1/2): Defining tables and classes (using MySQL)

Data definitions are specified only in the database. Active Record queries the database for the column names (that then serves to determine which attributes are valid) on regular object instantiation through the new constructor and relies on the column names in the rows with the finders.

       # CREATE TABLE companies (
       #   id int(11) unsigned NOT NULL auto_increment,
       #   client_of int(11),
       #   name varchar(255),
       #   type varchar(100),
       #   PRIMARY KEY  (id)
       # )

Active Record automatically links the "Company" object to the "companies" table

       class Company < ActiveRecord::Base
         has_many :people, :class_name => "Person"
       end

       class Firm < Company
         has_many :clients

         def people_with_all_clients
          clients.inject([]) { |people, client| people + client.people }
         end
       end

The foreign_key is only necessary because we didn‘t use "firm_id" in the data definition

       class Client < Company
         belongs_to :firm, :foreign_key => "client_of"
       end

       # CREATE TABLE people (
       #   id int(11) unsigned NOT NULL auto_increment,
       #   name text,
       #   company_id text,
       #   PRIMARY KEY  (id)
       # )

Active Record will also automatically link the "Person" object to the "people" table

       class Person < ActiveRecord::Base
         belongs_to :company
       end

## Simple example (2/2): Using the domain

Picking a database connection for all the Active Records

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

Create some fixtures

       firm = Firm.new("name" => "Next Angle")
       # SQL: INSERT INTO companies (name, type) VALUES("Next Angle", "Firm")
       firm.save

       client = Client.new("name" => "37signals", "client_of" => firm.id)
       # SQL: INSERT INTO companies (name, client_of, type) VALUES("37signals", 1, "Firm")
       client.save

Lots of different finders

       # SQL: SELECT * FROM companies WHERE id = 1
       next_angle = Company.find(1)

       # SQL: SELECT * FROM companies WHERE id = 1 AND type = 'Firm'
       next_angle = Firm.find(1)

       # SQL: SELECT * FROM companies WHERE id = 1 AND name = 'Next Angle'
       next_angle = Company.find(:first, :conditions => "name = 'Next Angle'")

       next_angle = Firm.find_by_sql("SELECT * FROM companies WHERE id = 1").first

The supertype, Company, will return subtype instances

       Firm === next_angle

All the dynamic methods added by the has_many macro

      next_angle.clients.empty?  # true
      next_angle.clients.size    # total number of clients
      all_clients = next_angle.clients

Constrained finds makes access security easier when ID comes from a web-app

       # SQL: SELECT * FROM companies WHERE client_of = 1 AND type = 'Client' AND id = 2
       thirty_seven_signals = next_angle.clients.find(2)

Bi-directional associations thanks to the "belongs_to" macro

       thirty_seven_signals.firm.nil? # true

## Philosophy

Active Record attempts to provide a coherent wrapper as a solution for the inconvenience that is object-relational mapping. The prime directive for this mapping has been to minimize the amount of code needed to build a real-world domain model. This is made possible by relying on a number of conventions that make it easy for Active Record to infer complex relations and structures from a minimal amount of explicit direction.

Convention over Configuration:

- No XML-files!
- Lots of reflection and run-time extension
- Magic is not inherently a bad word

Admit the Database:

- Lets you drop down to SQL for odd cases and performance
- Doesn‘t attempt to duplicate or replace data definitions

## Download

The latest version of Active Record can be found at

- rubyforge.org/project/showfiles.php?group_id=182

Documentation can be found at

- ar.rubyonrails.com

## Installation

The prefered method of installing Active Record is through its GEM file. You‘ll need to have RubyGems installed for that, though. If you have, then use:

      % [sudo] gem install activerecord-1.10.0.gem

You can also install Active Record the old-fashioned way with the following command:

      % [sudo] ruby install.rb

from its distribution directory.

## License

Active Record is released under the MIT license.

## Support

The Active Record homepage is www.rubyonrails.com. You can find the Active Record RubyForge page at rubyforge.org/projects/activerecord. And as Jim from Rake says:

       Feel free to submit commits or feature requests.  If you send a patch,
       remember to update the corresponding unit tests.  If fact, I prefer
       new feature to be submitted in the form of new unit tests.

For other information, feel free to ask on the rubyonrails-talk (groups.google.com/group/rubyonrails-talk) mailing list.

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