I’m incredibly proud to present the first public release of Active
Record – an implementation of the ActiveRecord pattern for
object-relational mapping. I’ve entitled this release Three-Quarters to
reflect the fact that Active Record has been in production use on
Basecamp for more than three months and used during the many months of
development before that. Additionally, it’s been used by around five
beta testers in a number of different projects. So even though I’m not
ready to declare it 1.0 out of the gates, it is indeed a solid release.
It also marks the first step towards releasing the web-application
framework Rails, which I’ve been hyping and talking about for some time
now. Active Record is the model part of Rails and contains around
two-thirds of the code in the framework. So if you want to get ready
for making Rails applications, it’s a good idea to familiarize yourself
with Active Record.
Further more, I’d like to extend my thanks to Luke Holden, Jamis Buck,
Aredidel, Guan Yang, and Lau Tårnskov for helping me make this release
a reality. All your help and suggestions have been much appreciated!
Below is a snapshot of the README. Find more information and links to
downloads on the official website at
http://activerecord.rubyonrails.org/ and on the API site at
http://ar.rubyonrails.org/
/ David Heinemeier Hansson
== 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 is presented in one wrapping. It’s an
implementation of the object-relational mapping (ORM) pattern 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 Records 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:
-
Associations between objects controlled by simple meta-programming
macros.
class Firm < ActiveRecord::Base
has_many :clients
has_one :account
belong_to :conglomorate
end -
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 -
Validation rules that can differ for new or existing objects.
class Post < ActiveRecord::Base
def validate # validates on both creates and updates
errors.add_on_empty "title"
enddef validate_on_update errors.add_on_empty "password" endend
-
Callbacks 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 -
Observers for the entire lifecycle
class CommentObserver < ActiveRecord::Observer
def after_create(comment) # is called just after Comment#save
NotificationService.send_email("david@loudthinking.com", comment)
end
end -
Inheritance hierarchies
class Company < ActiveRecord::Base; end
class Firm < Company; end
class Client < Company; end
class PriorityClient < Client; end -
Direct manipulation (instead of service invocation)
So instead of (Hibernate 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 a the same cat...
cat.save
-
Database abstraction for three different engines through simple
adapters
ActiveRecord::Base.mysql_connection(host, username, pass, database)
ActiveRecord::Base.postgresql_connection(host, table, username,
pass, database)
ActiveRecord::Base.sqlite_connection(dbfile) -
Logging support for Log4r and Logger
== Philosophy
Active Record attempts to provide a coherent wrapping for the
inconvenience that is object-relational mapping. The prime directive
for this mapping has been to minimize the amount of code needed to
built 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