2e81ca2d30
Updated to Rails 2.2.2. Added a couple more Ruby 1.9 fixes, but that's pretty much at a standstill, until one gets Maruku and HTML5lib working right under Ruby 1.9.
352 lines
11 KiB
Plaintext
352 lines
11 KiB
Plaintext
= Active Record -- Object-relation mapping put on rails
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Active Record connects business objects and database tables to create a persistable
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domain model where logic and data are presented in one wrapping. It's an implementation
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of the object-relational mapping (ORM) pattern[http://www.martinfowler.com/eaaCatalog/activeRecord.html]
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by the same name as described by Martin Fowler:
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"An object that wraps a row in a database table or view, encapsulates
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the database access, and adds domain logic on that data."
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Active Record's main contribution to the pattern is to relieve the original of two stunting problems:
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lack of associations and inheritance. By adding a simple domain language-like set of macros to describe
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the former and integrating the Single Table Inheritance pattern for the latter, Active Record narrows the
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gap of functionality between the data mapper and active record approach.
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A short rundown of the major features:
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* Automated mapping between classes and tables, attributes and columns.
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class Product < ActiveRecord::Base; end
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...is automatically mapped to the table named "products", such as:
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CREATE TABLE products (
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id int(11) NOT NULL auto_increment,
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name varchar(255),
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PRIMARY KEY (id)
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);
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...which again gives Product#name and Product#name=(new_name)
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{Learn more}[link:classes/ActiveRecord/Base.html]
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* Associations between objects controlled by simple meta-programming macros.
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class Firm < ActiveRecord::Base
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has_many :clients
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has_one :account
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belongs_to :conglomorate
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end
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{Learn more}[link:classes/ActiveRecord/Associations/ClassMethods.html]
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* Aggregations of value objects controlled by simple meta-programming macros.
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class Account < ActiveRecord::Base
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composed_of :balance, :class_name => "Money",
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:mapping => %w(balance amount)
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composed_of :address,
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:mapping => [%w(address_street street), %w(address_city city)]
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end
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{Learn more}[link:classes/ActiveRecord/Aggregations/ClassMethods.html]
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* Validation rules that can differ for new or existing objects.
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class Account < ActiveRecord::Base
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validates_presence_of :subdomain, :name, :email_address, :password
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validates_uniqueness_of :subdomain
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validates_acceptance_of :terms_of_service, :on => :create
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validates_confirmation_of :password, :email_address, :on => :create
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end
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{Learn more}[link:classes/ActiveRecord/Validations.html]
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* Callbacks as methods or queues on the entire lifecycle (instantiation, saving, destroying, validating, etc).
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class Person < ActiveRecord::Base
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def before_destroy # is called just before Person#destroy
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CreditCard.find(credit_card_id).destroy
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end
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end
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class Account < ActiveRecord::Base
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after_find :eager_load, 'self.class.announce(#{id})'
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end
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{Learn more}[link:classes/ActiveRecord/Callbacks.html]
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* Observers for the entire lifecycle
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class CommentObserver < ActiveRecord::Observer
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def after_create(comment) # is called just after Comment#save
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Notifications.deliver_new_comment("david@loudthinking.com", comment)
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end
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end
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{Learn more}[link:classes/ActiveRecord/Observer.html]
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* Inheritance hierarchies
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class Company < ActiveRecord::Base; end
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class Firm < Company; end
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class Client < Company; end
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class PriorityClient < Client; end
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{Learn more}[link:classes/ActiveRecord/Base.html]
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* Transactions
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# Database transaction
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Account.transaction do
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david.withdrawal(100)
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mary.deposit(100)
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end
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{Learn more}[link:classes/ActiveRecord/Transactions/ClassMethods.html]
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* Reflections on columns, associations, and aggregations
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reflection = Firm.reflect_on_association(:clients)
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reflection.klass # => Client (class)
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Firm.columns # Returns an array of column descriptors for the firms table
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{Learn more}[link:classes/ActiveRecord/Reflection/ClassMethods.html]
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* Direct manipulation (instead of service invocation)
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So instead of (Hibernate[http://www.hibernate.org/] example):
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long pkId = 1234;
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DomesticCat pk = (DomesticCat) sess.load( Cat.class, new Long(pkId) );
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// something interesting involving a cat...
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sess.save(cat);
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sess.flush(); // force the SQL INSERT
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Active Record lets you:
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pkId = 1234
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cat = Cat.find(pkId)
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# something even more interesting involving the same cat...
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cat.save
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{Learn more}[link:classes/ActiveRecord/Base.html]
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* Database abstraction through simple adapters (~100 lines) with a shared connector
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ActiveRecord::Base.establish_connection(:adapter => "sqlite", :database => "dbfile")
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ActiveRecord::Base.establish_connection(
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:adapter => "mysql",
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:host => "localhost",
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:username => "me",
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:password => "secret",
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:database => "activerecord"
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)
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{Learn more}[link:classes/ActiveRecord/Base.html#M000081] and read about the built-in support for
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MySQL[link:classes/ActiveRecord/ConnectionAdapters/MysqlAdapter.html], PostgreSQL[link:classes/ActiveRecord/ConnectionAdapters/PostgreSQLAdapter.html], SQLite[link:classes/ActiveRecord/ConnectionAdapters/SQLiteAdapter.html], Oracle[link:classes/ActiveRecord/ConnectionAdapters/OracleAdapter.html], SQLServer[link:classes/ActiveRecord/ConnectionAdapters/SQLServerAdapter.html], and DB2[link:classes/ActiveRecord/ConnectionAdapters/DB2Adapter.html].
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* Logging support for Log4r[http://log4r.sourceforge.net] and Logger[http://www.ruby-doc.org/stdlib/libdoc/logger/rdoc]
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ActiveRecord::Base.logger = Logger.new(STDOUT)
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ActiveRecord::Base.logger = Log4r::Logger.new("Application Log")
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* Database agnostic schema management with Migrations
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class AddSystemSettings < ActiveRecord::Migration
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def self.up
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create_table :system_settings do |t|
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t.string :name
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t.string :label
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t.text :value
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t.string :type
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t.integer :position
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end
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SystemSetting.create :name => "notice", :label => "Use notice?", :value => 1
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end
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def self.down
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drop_table :system_settings
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end
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end
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{Learn more}[link:classes/ActiveRecord/Migration.html]
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== Simple example (1/2): Defining tables and classes (using MySQL)
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Data definitions are specified only in the database. Active Record queries the database for
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the column names (that then serves to determine which attributes are valid) on regular
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object instantiation through the new constructor and relies on the column names in the rows
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with the finders.
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# CREATE TABLE companies (
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# id int(11) unsigned NOT NULL auto_increment,
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# client_of int(11),
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# name varchar(255),
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# type varchar(100),
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# PRIMARY KEY (id)
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# )
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Active Record automatically links the "Company" object to the "companies" table
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class Company < ActiveRecord::Base
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has_many :people, :class_name => "Person"
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end
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class Firm < Company
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has_many :clients
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def people_with_all_clients
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clients.inject([]) { |people, client| people + client.people }
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end
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end
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The foreign_key is only necessary because we didn't use "firm_id" in the data definition
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class Client < Company
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belongs_to :firm, :foreign_key => "client_of"
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end
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# CREATE TABLE people (
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# id int(11) unsigned NOT NULL auto_increment,
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# name text,
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# company_id text,
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# PRIMARY KEY (id)
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# )
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Active Record will also automatically link the "Person" object to the "people" table
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class Person < ActiveRecord::Base
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belongs_to :company
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end
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== Simple example (2/2): Using the domain
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Picking a database connection for all the Active Records
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ActiveRecord::Base.establish_connection(
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:adapter => "mysql",
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:host => "localhost",
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:username => "me",
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:password => "secret",
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:database => "activerecord"
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)
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Create some fixtures
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firm = Firm.new("name" => "Next Angle")
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# SQL: INSERT INTO companies (name, type) VALUES("Next Angle", "Firm")
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firm.save
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client = Client.new("name" => "37signals", "client_of" => firm.id)
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# SQL: INSERT INTO companies (name, client_of, type) VALUES("37signals", 1, "Firm")
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client.save
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Lots of different finders
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# SQL: SELECT * FROM companies WHERE id = 1
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next_angle = Company.find(1)
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# SQL: SELECT * FROM companies WHERE id = 1 AND type = 'Firm'
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next_angle = Firm.find(1)
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# SQL: SELECT * FROM companies WHERE id = 1 AND name = 'Next Angle'
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next_angle = Company.find(:first, :conditions => "name = 'Next Angle'")
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next_angle = Firm.find_by_sql("SELECT * FROM companies WHERE id = 1").first
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The supertype, Company, will return subtype instances
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Firm === next_angle
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All the dynamic methods added by the has_many macro
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next_angle.clients.empty? # true
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next_angle.clients.size # total number of clients
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all_clients = next_angle.clients
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Constrained finds makes access security easier when ID comes from a web-app
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# SQL: SELECT * FROM companies WHERE client_of = 1 AND type = 'Client' AND id = 2
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thirty_seven_signals = next_angle.clients.find(2)
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Bi-directional associations thanks to the "belongs_to" macro
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thirty_seven_signals.firm.nil? # true
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== Philosophy
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Active Record attempts to provide a coherent wrapper as a solution for the inconvenience that is
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object-relational mapping. The prime directive for this mapping has been to minimize
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the amount of code needed to build a real-world domain model. This is made possible
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by relying on a number of conventions that make it easy for Active Record to infer
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complex relations and structures from a minimal amount of explicit direction.
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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
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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
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== Download
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The latest version of Active Record can be found at
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* http://rubyforge.org/project/showfiles.php?group_id=182
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Documentation can be found at
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* http://ar.rubyonrails.com
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== Installation
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The prefered method of installing Active Record is through its GEM file. You'll need to have
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RubyGems[http://rubygems.rubyforge.org/wiki/wiki.pl] installed for that, though. If you have,
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then use:
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% [sudo] gem install activerecord-1.10.0.gem
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You can also install Active Record the old-fashioned way with the following command:
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% [sudo] ruby install.rb
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from its distribution directory.
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== License
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Active Record is released under the MIT license.
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== Support
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The Active Record homepage is http://www.rubyonrails.com. You can find the Active Record
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RubyForge page at http://rubyforge.org/projects/activerecord. And as Jim from Rake says:
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Feel free to submit commits or feature requests. If you send a patch,
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remember to update the corresponding unit tests. If fact, I prefer
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new feature to be submitted in the form of new unit tests.
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For other information, feel free to ask on the rubyonrails-talk
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(http://groups.google.com/group/rubyonrails-talk) mailing list.
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