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
```
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The Product class is automatically mapped to the table named “products”, which might look like this:

CREATE TABLE products (
  id bigint 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

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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

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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

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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

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Inheritance hierarchies.

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

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Transactions.

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

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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

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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 and read about the built-in support for MySQL, PostgreSQL, and SQLite3.

Logging support for Log4r and Logger.

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[5.0]
  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

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Philosophy

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: