= 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.

You can read more about Active Record in the {Active Record Basics}[https://edgeguides.rubyonrails.org/active_record_basics.html] guide.

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 bigint NOT NULL auto_increment,
     name varchar(255),
     PRIMARY KEY  (id)
   );

This would also define the following accessors: <tt>Product#name</tt> and
<tt>Product#name=(new_name)</tt>.


* 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/Mysql2Adapter.html],
  PostgreSQL[link:classes/ActiveRecord/ConnectionAdapters/PostgreSQLAdapter.html], and
  SQLite3[link:classes/ActiveRecord/ConnectionAdapters/SQLite3Adapter.html].


* Logging support for Log4r[https://github.com/colbygk/log4r] and Logger[https://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[6.0]
      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

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


== Philosophy

Active Record is an implementation of the object-relational mapping (ORM)
pattern[https://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:

  $ 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:

* https://opensource.org/licenses/MIT


== Support

API documentation is at:

* https://api.rubyonrails.org

Bug reports for the Ruby on Rails project can be filed here:

* https://github.com/rails/rails/issues

Feature requests should be discussed on the rails-core mailing list here:

* https://discuss.rubyonrails.org/c/rubyonrails-core