What's new in Active Record 0.9.0?
···
==================================
It’s been a while, but the wait will be worth it. This massive update includes thread safety, 400% speed increase (on a 100 objects loop), better callback definitions, natural association assignment, hashes and arrays in YAML storage, and much, much more.
Download from http://activerecord.rubyonrails.org, talk on #rubyonrails (FreeNet).
* Active Record is now thread safe! (So you can use it with Cerise and WEBrick
applications) [Implementation idea by Michael Neumann, debugging assistance
by Jamis Buck]
* Improved performance by roughly 400% on a basic test case of pulling 100
records and querying one attribute. This brings the tax for using Active
Record instead of "riding on the metal" (using MySQL-ruby C-driver directly)
down to ~50%. Done by doing lazy type conversions and caching column
information on the class-level.
* Added callback objects and procs as options for implementing the target for
callback macros.
* Added "counter_cache" option to belongs_to that automates the usage of
increment_counter and decrement_counter. Consider:
class Post < ActiveRecord::Base
has_many :comments
end
class Comment < ActiveRecord::Base
belongs_to :post
end
Iterating over 100 posts like this:
<% for post in @posts %>
<%= post.title %> has <%= post.comments_count %> comments
<% end %>
Will generate 100 SQL count queries -- one for each call to
post.comments_count. If you instead add a "comments_count" int column
to the posts table and rewrite the comments association macro with:
class Comment < ActiveRecord::Base
belongs_to :post, :counter_cache => true
end
Those 100 SQL count queries will be reduced to zero. Beware that counter
caching is only appropriate for objects that begin life
with the object it's specified to belong with and is destroyed like that as
well. Typically objects where you would also specify :dependent => true. If
your objects switch from one belonging to another (like a post that can be
move from one category to another), you'll have to manage the counter
yourself.
* Added natural object-style assignment for has_one and belongs_to
associations. Consider the following model:
class Project < ActiveRecord::Base
has_one :manager
end
class Manager < ActiveRecord::Base
belongs_to :project
end
Earlier, assignments would work like following regardless of which way the
assignment told the best story:
active_record.manager_id = david.id
Now you can do it either from the belonging side:
david.project = active_record
...or from the having side:
active_record.manager = david
If the assignment happens from the having side, the assigned object is
automatically saved. So in the example above, the project_id attribute on
david would be set to the id of active_record, then david would be saved.
* Added natural object-style assignment for has_many associations [Florian
Weber]. Consider the following model:
class Project < ActiveRecord::Base
has_many :milestones
end
class Milestone < ActiveRecord::Base
belongs_to :project
end
Earlier, assignments would work like following regardless of which way the
assignment told the best story:
deadline.project_id = active_record.id
Now you can do it either from the belonging side:
deadline.project = active_record
...or from the having side:
active_record.milestones << deadline
The milestone is automatically saved with the new foreign key.
* API CHANGE: Attributes for text (or blob or similar) columns will now have
unknown classes stored using YAML instead of using to_s. (Known classes that
won't be yamelized are: String, NilClass, TrueClass, FalseClass, Fixnum,
Date, and Time). Likewise, data pulled out of text-based attributes will be
attempted converged using Yaml if they have the "--- " header. This was
primarily done to be enable the storage of hashes and arrays without
wrapping them in aggregations, so now you can do:
user = User.find(1)
user.preferences = { "background" => "black", "display" => large }
user.save
User.find(1).preferences # => { "background"=>"black", "display"=>large }
Please note that this method should only be used when you don't care about
representing the object in proper columns in the database. A money object
consisting of an amount and a currency is still a much better fit for a
value object done through aggregations than this new option.
* POSSIBLE CODE BREAKAGE: As a consequence of the lazy type conversions, it's
a bad idea to reference the @attributes hash directly (it always was, but
now it's paramount that you don't). If you do, you won't get the type
conversion. So to implement new accessors for existing attributes, use
read_attribute(attr_name) and write_attribute(attr_name, value) instead.
Like this:
class Song < ActiveRecord::Base
# Uses an integer of seconds to hold the length of the song
def length=(minutes)
write_attribute("length", minutes * 60)
end
def length
read_attribute("length") / 60
end
end
The clever kid will notice that this opens a door to sidestep the automated
type conversion by using @attributes directly. This is not recommended as
read/write_attribute may be granted additional responsibilities in the
future, but if you think you know what you're doing and aren't afraid of
future consequences, this is an option.
* Applied a few minor bug fixes reported by Daniel Von Fange.
What about Active Record 1.0.0?
Active Record will be moving to promised land of 1.0.0 within a reasonably short time frame. So if you have any wishes, comments, or complaints, you'll want to voice them sooner rather than later. 1.0.0 won't mean the end of developement, of course, but it would be nice to have a really solid release. So do speak forth.
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("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 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
--
David Heinemeier Hansson,
http://www.instiki.org/ -- A No-Step-Three Wiki in Ruby
http://www.basecamphq.com/ -- Web-based Project Management
http://www.loudthinking.com/ -- Broadcasting Brain
http://www.nextangle.com/ -- Development & Consulting Services
