From: David Heinemeier Hansson <david@...>
Date: 2004-06-03T00:09:23+09:00
Subject: [ANN] Active Record 0.8.0: Transaction that!

What's new in Active Record 0.8.0?
==================================

Transactions are here! This was the last major feature destined for  
inclusion before version 1.0 is released. So I was happy to see that it  
could be done in just a handful of lines. Transactions are guarded by  
blocks and work as such:

     Account.transaction do
       david.withdrawal(100)
       mary.deposit(100)
     end

This example will only take money from David and give to Mary if  
neither withdrawal nor deposit raises an exception. Exceptions (and  
returning false in the last statement of the transaction block) will  
force a ROLLBACK that returns the database to the state before the  
transaction was begun.

Be aware, though, that the objects will not have their instance data  
returned to their state before the transaction started. You�ll have to  
deal with that yourself (just as in the case of Validations). Also have  
in mind that exceptions thrown within a transaction block will be  
propagated (after triggering the ROLLBACK), so you should be ready to  
catch those in your application code.

Also in 0.8.0:

* Changed Base.find to also accept either a list (1, 5, 6) or an array  
of ids ([5, 7])
   as parameter and then return an array of objects instead of just an  
object

* Fixed method has_collection? for has_and_belongs_to_many macro to  
behave as a
   collection, not an association

* Fixed SQLite adapter so empty or nil values in columns of datetime,  
date, or time type
   aren't treated as current time [Spotted by Gavin Sinclair]

Get the release and read more at http://activerecord.rubyonrails.org/


Call for help!
==============

Do you have working knowledge with and access to either Oracle, ODBC,  
Sybase, or DB2, I'd be really grateful if you would consider writing an  
adapter for Active Record. Adapters are usually just around 100 lines  
of code. You'll have three examples to look at, a well-specified  
interface[1], and almost 100 test cases to make it real easy. Luke  
Holden reports that he spent just a few hours getting SQLite and  
PostgreSQL adapters working.

[1]  
http://ar.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/ 
AbstractAdapter.html


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:

* 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)


* 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"
      end

      def validate_on_update
        errors.add_on_empty "password"
      end
    end


* 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


* Transaction support
     Account.transaction do
       david.withdrawal(100)
       mary.deposit(100)
     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