Active Record – Object-relational mapping put on rails
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 the 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:
Automated mapping between classes and tables, attributes and columns.
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)`
Associations between objects defined by simple class methods.
class Firm < ActiveRecord::Base has_many :clients has_one :account belongs_to :conglomerate end
Aggregations of value objects.
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 :subdomain, :name, :email_address, :password, presence: true validates :subdomain, uniqueness: true validates :terms_of_service, acceptance: true, on: :create validates :password, :email_address, confirmation: true, on: :create end
Callbacks available for the entire life cycle (instantiation, saving, destroying, validating, etc.).
class Person < ActiveRecord::Base before_destroy :invalidate_payment_plan # the `invalidate_payment_plan` method gets called just before Person#destroy end
Observers that react to changes in a model.
class CommentObserver < ActiveRecord::Observer def after_create(comment) # is called just after Comment#save CommentMailer.new_comment_email('firstname.lastname@example.org', comment).deliver 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
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
Database abstraction through simple adapters.
# connect to SQLite3 ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: 'dbfile.sqlite3') # connect to MySQL with authentication ActiveRecord::Base.establish_connection( adapter: 'mysql2', host: 'localhost', username: 'me', password: 'secret', database: 'activerecord' )
ActiveRecord::Base.logger = ActiveSupport::Logger.new(STDOUT) ActiveRecord::Base.logger = Log4r::Logger.new('Application Log')
Database agnostic schema management with Migrations.
class AddSystemSettings < ActiveRecord::Migration def 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 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:
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 and installation
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: