Path: | README.rdoc |
Last Update: | Thu Jan 10 09:45:36 -0500 2013 |
Active Record connects classes to relational database tables to establish an almost zero-configuration persistence layer for applications. The library provides a base class that, when subclassed, sets up a mapping between the new class and an existing table in the database. In context of an application, these classes are commonly referred to as models. Models can also be connected to other models; this is done by defining associations.
Active Record relies heavily on naming in that it uses class and association names to establish mappings between respective database tables and foreign key columns. Although these mappings can be defined explicitly, it‘s recommended to follow naming conventions, especially when getting started with the library.
A short rundown of some of the major features:
class Product < ActiveRecord::Base end The Product class is automatically mapped to the table named "products", which might look like this: CREATE TABLE products ( id int(11) NOT NULL auto_increment, name varchar(255), PRIMARY KEY (id) ); This would also define the following accessors: `Product#name` and `Product#name=(new_name)`
class Firm < ActiveRecord::Base has_many :clients has_one :account belongs_to :conglomerate end
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
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
class Person < ActiveRecord::Base before_destroy :invalidate_payment_plan # the `invalidate_payment_plan` method gets called just before Person#destroy end
class CommentObserver < ActiveRecord::Observer def after_create(comment) # is called just after Comment#save Notifications.deliver_new_comment("david@loudthinking.com", comment) end end
class Company < ActiveRecord::Base; end class Firm < Company; end class Client < Company; end class PriorityClient < Client; end
# Database transaction Account.transaction do david.withdrawal(100) mary.deposit(100) end
reflection = Firm.reflect_on_association(:clients) reflection.klass # => Client (class) Firm.columns # Returns an array of column descriptors for the firms table
# connect to SQLite3 ActiveRecord::Base.establish_connection(:adapter => "sqlite3", :database => "dbfile.sqlite3") # connect to MySQL with authentication 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, and SQLite3.
ActiveRecord::Base.logger = Logger.new(STDOUT) ActiveRecord::Base.logger = Log4r::Logger.new("Application Log")
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
Active Record is an implementation of the object-relational mapping (ORM) pattern by the same name 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 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:
Admit the Database:
The latest version of Active Record can be installed with Rubygems:
% [sudo] gem install activerecord
Source code can be downloaded as part of the Rails project on GitHub
Active Record is released under the MIT license.
API documentation is at
Bug reports and feature requests can be filed with the rest for the Ruby on Rails project here: