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= Active Record -- Object-relation mapping put on rails Active Record connects business objects and database tables to create a persistable domain model where logic and data are presented in one wrapping. It's an implementation of the object-relational mapping (ORM) pattern[http://www.martinfowler.com/eaaCatalog/activeRecord.html] by the same name as 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's main contribution to the pattern is to relieve the original of two stunting problems: lack of associations and inheritance. By adding a simple domain language-like set of macros to describe the former and integrating the Single Table Inheritance pattern for the latter, Active Record narrows the gap of functionality between the data mapper and active record approach. A short rundown of the major features: * Automated mapping between classes and tables, attributes and columns. class Product < ActiveRecord::Base; end ...is automatically mapped to the table named "products", such as: CREATE TABLE products ( id int(11) NOT NULL auto_increment, name varchar(255), PRIMARY KEY (id) ); ...which again gives Product#name and Product#name=(new_name) {Learn more}[link:classes/ActiveRecord/Base.html] * Associations between objects controlled by simple meta-programming macros. class Firm < ActiveRecord::Base has_many :clients has_one :account belongs_to :conglomorate end {Learn more}[link:classes/ActiveRecord/Associations/ClassMethods.html] * Aggregations of value objects controlled by simple meta-programming macros. 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_presence_of :subdomain, :name, :email_address, :password validates_uniqueness_of :subdomain validates_acceptance_of :terms_of_service, :on => :create validates_confirmation_of :password, :email_address, :on => :create end {Learn more}[link:classes/ActiveRecord/Validations.html] * Acts that can make records work as lists or trees: class Item < ActiveRecord::Base belongs_to :list acts_as_list :scope => :list end item.move_higher item.move_to_bottom Learn about {acts_as_list}[link:classes/ActiveRecord/Acts/List/ClassMethods.html], {the instance methods acts_as_list provides}[link:classes/ActiveRecord/Acts/List/InstanceMethods.html], and {acts_as_tree}[link:classes/ActiveRecord/Acts/Tree/ClassMethods.html] * Callbacks as methods or queues on the entire lifecycle (instantiation, saving, destroying, validating, etc). class Person < ActiveRecord::Base def before_destroy # is called just before Person#destroy CreditCard.find(credit_card_id).destroy end end class Account < ActiveRecord::Base after_find :eager_load, 'self.class.announce(#{id})' end {Learn more}[link:classes/ActiveRecord/Callbacks.html] * Observers for the entire lifecycle class CommentObserver < ActiveRecord::Observer def after_create(comment) # is called just after Comment#save Notifications.deliver_new_comment("david@loudthinking.com", comment) end end {Learn more}[link:classes/ActiveRecord/Observer.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] * Transaction support on both a database and object level. The latter is implemented by using Transaction::Simple[http://www.halostatue.ca/ruby/Transaction__Simple.html] # Just database transaction Account.transaction do david.withdrawal(100) mary.deposit(100) end # Database and object transaction Account.transaction(david, mary) 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] * Direct manipulation (instead of service invocation) So instead of (Hibernate[http://www.hibernate.org/] example): long pkId = 1234; DomesticCat pk = (DomesticCat) sess.load( Cat.class, new Long(pkId) ); // something interesting involving a cat... sess.save(cat); sess.flush(); // force the SQL INSERT Active Record lets you: pkId = 1234 cat = Cat.find(pkId) # something even more interesting involving the same cat... cat.save {Learn more}[link:classes/ActiveRecord/Base.html] * Database abstraction through simple adapters (~100 lines) with a shared connector ActiveRecord::Base.establish_connection(:adapter => "sqlite", :database => "dbfile") ActiveRecord::Base.establish_connection( :adapter => "mysql", :host => "localhost", :username => "me", :password => "secret", :database => "activerecord" ) {Learn more}[link:classes/ActiveRecord/Base.html#M000081] and read about the built-in support for 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/OCIAdapter.html], SQLServer[link:classes/ActiveRecord/ConnectionAdapters/SQLServerAdapter.html], and DB2[link:classes/ActiveRecord/ConnectionAdapters/DB2Adapter.html]. * Logging support for Log4r[http://log4r.sourceforge.net] and Logger[http://www.ruby-doc.org/stdlib/libdoc/logger/rdoc] ActiveRecord::Base.logger = Logger.new(STDOUT) ActiveRecord::Base.logger = Log4r::Logger.new("Application Log") == Simple example (1/2): Defining tables and classes (using MySQL) Data definitions are specified only in the database. Active Record queries the database for the column names (that then serves to determine which attributes are valid) on regular object instantiation through the new constructor and relies on the column names in the rows with the finders. # CREATE TABLE companies ( # id int(11) unsigned NOT NULL auto_increment, # client_of int(11), # name varchar(255), # type varchar(100), # PRIMARY KEY (id) # ) Active Record automatically links the "Company" object to the "companies" table class Company < ActiveRecord::Base has_many :people, :class_name => "Person" end class Firm < Company has_many :clients def people_with_all_clients clients.inject([]) { |people, client| people + client.people } end end The foreign_key is only necessary because we didn't use "firm_id" in the data definition class Client < Company belongs_to :firm, :foreign_key => "client_of" end # CREATE TABLE people ( # id int(11) unsigned NOT NULL auto_increment, # name text, # company_id text, # PRIMARY KEY (id) # ) Active Record will also automatically link the "Person" object to the "people" table class Person < ActiveRecord::Base belongs_to :company end == Simple example (2/2): Using the domain Picking a database connection for all the Active Records ActiveRecord::Base.establish_connection( :adapter => "mysql", :host => "localhost", :username => "me", :password => "secret", :database => "activerecord" ) Create some fixtures firm = Firm.new("name" => "Next Angle") # SQL: INSERT INTO companies (name, type) VALUES("Next Angle", "Firm") firm.save client = Client.new("name" => "37signals", "client_of" => firm.id) # SQL: INSERT INTO companies (name, client_of, type) VALUES("37signals", 1, "Firm") client.save Lots of different finders # SQL: SELECT * FROM companies WHERE id = 1 next_angle = Company.find(1) # SQL: SELECT * FROM companies WHERE id = 1 AND type = 'Firm' next_angle = Firm.find(1) # SQL: SELECT * FROM companies WHERE id = 1 AND name = 'Next Angle' next_angle = Company.find_first "name = 'Next Angle'" next_angle = Firm.find_by_sql("SELECT * FROM companies WHERE id = 1").first The supertype, Company, will return subtype instances Firm === next_angle All the dynamic methods added by the has_many macro next_angle.clients.empty? # true next_angle.clients.size # total number of clients all_clients = next_angle.clients Constrained finds makes access security easier when ID comes from a web-app # SQL: SELECT * FROM companies WHERE client_of = 1 AND type = 'Client' AND id = 2 thirty_seven_signals = next_angle.clients.find(2) Bi-directional associations thanks to the "belongs_to" macro thirty_seven_signals.firm.nil? # true == Examples Active Record ships with a couple of examples that should give you a good feel for operating usage. Be sure to edit the <tt>examples/shared_setup.rb</tt> file for your own database before running the examples. Possibly also the table definition SQL in the examples themselves. It's also highly recommended to have a look at the unit tests. Read more in link:files/RUNNING_UNIT_TESTS.html == Philosophy 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 The latest version of Active Record can be found at * http://rubyforge.org/project/showfiles.php?group_id=182 Documentation can be found at * http://ar.rubyonrails.com == Installation The prefered method of installing Active Record is through its GEM file. You'll need to have RubyGems[http://rubygems.rubyforge.org/wiki/wiki.pl] installed for that, though. If you have, then use: % [sudo] gem install activerecord-1.10.0.gem You can also install Active Record the old-fashion way with the following command: % [sudo] ruby install.rb from its distribution directory. == License Active Record is released under the MIT license. == Support The Active Record homepage is http://www.rubyonrails.com. You can find the Active Record RubyForge page at http://rubyforge.org/projects/activerecord. And as Jim from Rake says: Feel free to submit commits or feature requests. If you send a patch, remember to update the corresponding unit tests. If fact, I prefer new feature to be submitted in the form of new unit tests. For other information, feel free to ask on the ruby-talk mailing list (which is mirrored to comp.lang.ruby) or contact mailto:david@loudthinking.com.