= Active Record -- Object-relational mapping in 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
{Learn more}[link:classes/ActiveRecord/Base.html]
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
{Learn more}[link:classes/ActiveRecord/Associations/ClassMethods.html]
* 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
{Learn more}[link:classes/ActiveRecord/Aggregations/ClassMethods.html]
* 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
{Learn more}[link:classes/ActiveRecord/Validations.html]
* 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
{Learn more}[link:classes/ActiveRecord/Callbacks.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]
* 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'
)
{Learn more}[link:classes/ActiveRecord/Base.html] and read about the built-in support for
MySQL[link:classes/ActiveRecord/ConnectionAdapters/MysqlAdapter.html],
PostgreSQL[link:classes/ActiveRecord/ConnectionAdapters/PostgreSQLAdapter.html], and
SQLite3[link:classes/ActiveRecord/ConnectionAdapters/SQLite3Adapter.html].
* Logging support for Log4r[http://log4r.rubyforge.org] and Logger[http://www.ruby-doc.org/stdlib/libdoc/logger/rdoc].
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
{Learn more}[link:classes/ActiveRecord/Migration.html]
== Philosophy
Active Record is an implementation of the object-relational mapping (ORM)
pattern[http://www.martinfowler.com/eaaCatalog/activeRecord.html] 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:
* 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 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:
* https://github.com/rails/rails/tree/master/activerecord
== License
Active Record is released under the MIT license:
* http://www.opensource.org/licenses/MIT
== Support
API documentation is at:
* http://api.rubyonrails.org
Bug reports and feature requests can be filed with the rest for the Ruby on Rails project here:
* https://github.com/rails/rails/issues
