= 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[http://www.martinfowler.com/eaaCatalog/activeRecord.html] 
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}[link:classes/ActiveRecord/Base.html]


* 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}[link:classes/ActiveRecord/Associations/ClassMethods.html]


* 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}[link:classes/ActiveRecord/Aggregations/ClassMethods.html]


* 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}[link:classes/ActiveRecord/Validations.html]
 
* 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}[link:classes/ActiveRecord/Callbacks.html]


* 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}[link:classes/ActiveRecord/Observer.html]


* Inheritance hierarchies 

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

  {Learn more}[link:classes/ActiveRecord/Base.html]


* Transactions

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

  {Learn more}[link:classes/ActiveRecord/Transactions/ClassMethods.html]


* 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}[link:classes/ActiveRecord/Reflection/ClassMethods.html]


* Direct manipulation (instead of service invocation)

  So instead of (Hibernate[http://www.hibernate.org/] 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}[link:classes/ActiveRecord/Base.html]


* 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}[link:classes/ActiveRecord/Base.html#M000081] and read about the built-in support for
  MySQL[link:classes/ActiveRecord/ConnectionAdapters/MysqlAdapter.html], PostgreSQL[link:classes/ActiveRecord/ConnectionAdapters/PostgreSQLAdapter.html], SQLite[link:classes/ActiveRecord/ConnectionAdapters/SQLiteAdapter.html], Oracle[link:classes/ActiveRecord/ConnectionAdapters/OracleAdapter.html], SQLServer[link:classes/ActiveRecord/ConnectionAdapters/SQLServerAdapter.html], and DB2[link:classes/ActiveRecord/ConnectionAdapters/DB2Adapter.html].


* Logging support for Log4r[http://log4r.sourceforge.net] and Logger[http://www.ruby-doc.org/stdlib/libdoc/logger/rdoc]

    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}[link:classes/ActiveRecord/Migration.html]

== 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 installed with Rubygems:

* gem install activerecord

Documentation can be found at 

* http://api.rubyonrails.org


== License

Active Record is released under the MIT license.


== Support

The Active Record homepage is http://www.rubyonrails.com. You can find the Active Record
RubyForge page at http://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 
(http://groups.google.com/group/rubyonrails-talk) mailing list.