[ANN] Active Record 0.8.4: Reflections

  • Thread starter David Heinemeier Hansson
  • Start date
D

David Heinemeier Hansson

What's new in Active Record 0.8.4?
==================================

Reflection is the big new feature in 0.8.4. You're now able to get
information about the columns mapped to attributes, the names of all
the attributes pulled, and full information about both aggregations and
associations added. All this information is particularly helpful if
you're trying to build generic handlers for Active Record objects, such
as, say, a form builder that automatically lists the proper input field
types and names.

Besides that, it's now possible to do multi-parameter assigns that can
even be type-casted. This is extremely helpful to do something like
this for date attributes:

post.attributes = {
"written_on(1i)" => "2004",
"written_on(2i)" => "6",
"written_on(3i)" => "16"
}

If the written_on attribute is of the date type, this is turned into:

post.written_on = Date.new(2004, 6, 16)

The number in the parentheses represents the position in the
constructor and the i represents the type cast from string to integer
(you can also cast to floats or arrays).

Full changelog:

Reflection
----------

* Added ActiveRecord::Reflection with a bunch of methods and classes
for reflecting in aggregations and associations.

* Added Base.columns and Base.content_columns which returns arrays of
column description (type, default, etc) objects.

* Added Base#attribute_names which returns an array of names for the
attributes available on the object.

* Added Base#column_for_attribute(name) which returns the column
description object for the named attribute.


Misc
----

* Added multi-parameter assignment.

* Fixed bug with custom primary key column name and Base.find on
multiple parameters.

* Fixed bug with dependent option on has_one associations if there was
no associated object.


Hang out with the Ruby on Rails crowd
=====================================

Come by the IRC channel #rubyonrails on Freenode. Design decisions are
aired here and you'll be able to ask questions about Active Record and
the framework in general. Oh, and we're really friendly too!


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 as methods or ques 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 in link:classes/ActiveRecord/Callbacks.html


* Observers for the entire lifecycle
class CommentObserver < ActiveRecord::Observer
def after_create(comment) # is called just after Comment#save
NotificationService.send_email("(e-mail address removed)", comment)
end
end


* Inheritance hierarchies
class Company < ActiveRecord::Base; end
class Firm < Company; end
class Client < Company; end
class PriorityClient < Client; end


* Transaction support on both a database and object level. The latter
is implemented
by using Transaction::Simple

# 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


* 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 through simple adapters (~100 lines) with a
shared connector

ActiveRecord::Base.establish_connection:)adapter => "sqlite",
:dbfile => "dbfile")

ActiveRecord::Base.establish_connection(
:adapter => "mysql",
:host => "localhost",
:username => "me",
:password => "secret",
:database => "activerecord"
)


* Logging support for Log4r and Logger

ActiveRecord::Base.logger = Logger.new(STDOUT)
ActiveRecord::Base.logger = Log4r::Logger.new("Application Log")


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
 

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