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 guide.
A short rundown of some of the major features:
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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 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).
ActiveRecord::Base
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Associations between objects defined by simple class methods.
class Firm < ActiveRecord::Base has_many :clients has_one :account belongs_to :conglomerate end
ActiveRecord::Associations::ClassMethods
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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
ActiveRecord::Aggregations::ClassMethods
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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
ActiveRecord::Validations
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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
ActiveRecord::Callbacks
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Inheritance hierarchies.
class Company < ActiveRecord::Base; end class Firm < Company; end class Client < Company; end class PriorityClient < Client; end
ActiveRecord::Base
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# Database transaction Account.transaction do david.withdrawal(100) mary.deposit(100) end
ActiveRecord::Transactions::ClassMethods
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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
ActiveRecord::Reflection::ClassMethods
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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' )
ActiveRecord::Base and read about the built-in support for ActiveRecord::ConnectionAdapters::Mysql2Adapter ActiveRecord::ConnectionAdapters::PostgreSQLAdapter and ActiveRecord::ConnectionAdapters::SQLite3Adapter.
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Logging support for Log4r and Logger.
ActiveRecord::Base.logger = ActiveSupport::Logger.new(STDOUT) ActiveRecord::Base.logger = Log4r::Logger.new('Application Log')
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Database agnostic schema management with Migrations.
class AddSystemSettings < ActiveRecord::Migration[7.1] 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
ActiveRecord::Migration
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:
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No XML files!
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Lots of reflection and run-time extension
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Magic is not inherently a bad word
Admit the Database:
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Lets you drop down to SQL for odd cases and performance
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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:
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github.com/rails/rails/tree/main/activerecord
License
Active Record is released under the MIT license:
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opensource.org/licenses/MIT
Support
API documentation is at:
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api.rubyonrails.org
Bug reports for the Ruby on Rails project can be filed here:
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github.com/rails/rails/issues
Feature requests should be discussed on the rails-core mailing list here:
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discuss.rubyonrails.org/c/rubyonrails-core
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Constants
ORIGINAL_ATTRIBUTE_PREFIX = "original_"
UnknownAttributeError = ActiveModel::UnknownAttributeError
DeprecatedActiveJobRequiredError = Class.new(ActiveRecordError)
MigrationProxy = Struct.new(:name, :version, :filename, :scope) do def initialize(name, version, filename, scope) super @migration = nil end def basename File.basename(filename) end delegate :migrate, :announce, :write, :disable_ddl_transaction, to: :migration private def migration @migration ||= load_migration end def load_migration Object.send(:remove_const, name) rescue nil load(File.expand_path(filename)) name.constantize.new(name, version) end end
Point = Struct.new(:x, :y)
Attributes
[RW] | reading_role | |
[RW] | writing_role | |
[RW] | db_warnings_ignore | |
[R] | db_warnings_action | |
[R] | default_timezone | |
[RW] | schema_cache_ignored_tables | |
[RW] | lazily_load_schema_cache | |
[RW] | disable_prepared_statements |
The docs are in AR::Base
The docs you’re looking for are in ActiveRecord::Base