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[ANN] Active Record 0.8.3: Modules, mapping, and transactions

 
 
David Heinemeier Hansson
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      06-09-2004
What's new in Active Record 0.8.3?
==================================

Active Records in modules are now treated nicely by both the mapping
and association logic. The mapping can be more finely controlled with
options for different primary keys than “id” and for non-integer keys.
The dependent option on associations are now treated as a transaction.
And many other changes:


Transactions
------------

* Added transactional protection for destroy
(important for the new :dependent option) [Suggested by Carl
Youngblood]

* Fixed so transactions are ignored on MyISAM tables for MySQL
(use InnoDB to get transactions)

* Changed transactions so only exceptions will cause a rollback, not
returned false.


Mapping
-------

* Added support for non-integer primary keys [Aredridel/earlier work by
Michael Neumann]

User.find "jdoe"
Product.find "PDKEY-INT-12"

* Added option to specify naming method for primary key column.
ActiveRecord::Base.primary_key_prefix_type can either be set to nil,
:table_name, or
:table_name_with_underscore. :table_name will assume that Product
class has a primary key
of "productid" and :table_name_with_underscore will assume
"product_id".
The default nil will just give "id".

* Added an overwriteable primary_key method that'll instruct AR to the
name of the
id column [Aredridele/earlier work by Guan Yang]

class Project < ActiveRecord::Base
def self.primary_key() "project_id" end
end

* Fixed that Active Records can safely associate inside and out of
modules.

class MyApplication::Account < ActiveRecord::Base
has_many :clients # will look for MyApplication::Client
has_many :interests, :class_name => "Business::Interest" # will
look for Business::Interest
end

* Fixed that Active Records can safely live inside modules [Aredridel]

class MyApplication::Account < ActiveRecord::Base
end


Misc
----

* Added freeze call to value object assignments to ensure they remain
immutable
[Spotted by Gavin Sinclair]

* Changed interface for specifying observed class in observers. Was
OBSERVED_CLASS constant,
now is observed_class() class method. This is more consistant with
things like
self.table_name(). Works like this:

class AuditObserver < ActiveRecord::Observer
def self.observed_class() Account end
def after_update(account)
AuditTrail.new(account, "UPDATED")
end
end

[Suggested by Gavin Sinclair]

* Create new Active Record objects by setting the attributes through a
block. Like this:

person = Person.new do |p|
p.name = 'Freddy'
p.age = 19
end

[Suggested by Gavin Sinclair]

Get the new version on http://activerecord.rubyonrails.org or, even
better, type "gem -Ri activerecord"
when the 0.8.3 version appears on the "gem -Rl" list (it should very
soon).


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/Ac...ctionAdapters/
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",
assword => "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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Raphael Bauduin
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Posts: n/a
 
      06-09-2004
David Heinemeier Hansson wrote:
> What's new in Active Record 0.8.3?
> ==================================
>
> Active Records in modules are now treated nicely by both the mapping
> and association logic. The mapping can be more finely controlled with
> options for different primary keys than “id” and for non-integer keys.
> The dependent option on associations are now treated as a transaction.
> And many other changes:
>
>
> Transactions
> ------------
>
> * Added transactional protection for destroy
> (important for the new :dependent option) [Suggested by Carl Youngblood]
>
> * Fixed so transactions are ignored on MyISAM tables for MySQL
> (use InnoDB to get transactions)
>
> * Changed transactions so only exceptions will cause a rollback, not
> returned false.
>
>
> Mapping
> -------
>
> * Added support for non-integer primary keys [Aredridel/earlier work by
> Michael Neumann]
>
> User.find "jdoe"
> Product.find "PDKEY-INT-12"
>
> * Added option to specify naming method for primary key column.
> ActiveRecord::Base.primary_key_prefix_type can either be set to nil,
> :table_name, or
> :table_name_with_underscore. :table_name will assume that Product
> class has a primary key
> of "productid" and :table_name_with_underscore will assume "product_id".
> The default nil will just give "id".
>



Hi,

Just tested it. It works find when you want only on object return, but fails when you do this:

ua = User.find(8,9,10)

The cause, from the output I got, is that the field used to order the returned rows is still name 'id'

Raph
 
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Kirk Haines
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Posts: n/a
 
      06-09-2004
On Wed, 9 Jun 2004 21:15:37 +0900, David Heinemeier Hansson wrote

> * Fixed so transactions are ignored on MyISAM tables for MySQL
> (use InnoDB to get transactions)


David, I'm curious about this design decision. I went the exact opposite
direction with transactions in Kansas. Regardless of whether transactions
are implemented at the DBD layer or by the database, transactions are always
implemented at the object level. I find it interesting that you went the
other way, and am curious about the reasoning behind it?


Thanks. Active Record looks like it is shaping up nicely!


Kirk Haines



 
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Carl Youngblood
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Posts: n/a
 
      06-09-2004
Kirk Haines wrote:

>On Wed, 9 Jun 2004 21:15:37 +0900, David Heinemeier Hansson wrote
>
>>* Fixed so transactions are ignored on MyISAM tables for MySQL
>> (use InnoDB to get transactions)
>>
>>

>
>David, I'm curious about this design decision. I went the exact opposite
>direction with transactions in Kansas. Regardless of whether transactions
>are implemented at the DBD layer or by the database, transactions are always
>implemented at the object level. I find it interesting that you went the
>other way, and am curious about the reasoning behind it?
>

Another option worth considering is using LOCK TABLES for MyISAM
tables. MySQL's argument on this is that the MyISAM engine is about 3
times faster than the transactional engines and for those times when you
need more than one query to accomplish your goals, locking the tables
doesn't cause you to incur a large performance penalty. It still
doesn't protect you from a server outage or something but otherwise it
works pretty well.

I can imagine some scenarios where you might end up with inconsistent
data if you don't lock tables before performing a series of queries.

Carl


 
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David Heinemeier Hansson
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Posts: n/a
 
      06-09-2004
> David, I'm curious about this design decision. I went the exact
> opposite
> direction with transactions in Kansas. Regardless of whether
> transactions
> are implemented at the DBD layer or by the database, transactions are
> always
> implemented at the object level. I find it interesting that you went
> the
> other way, and am curious about the reasoning behind it?


Oh, this was referring to database transactions. Before, if you
attempted to create a database transaction block for MyISAM tables, it
would bork. Now it'll just ignore MySQL's complaints.

The object transactions are separate from this and require that you
name the objects going into that with the transaction call.

> Thanks. Active Record looks like it is shaping up nicely!


Thank you..
--
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



 
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David Heinemeier Hansson
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Posts: n/a
 
      06-09-2004
> Just tested it. It works find when you want only on object return, but
> fails when you do this:
>
> ua = User.find(8,9,10)
>
> The cause, from the output I got, is that the field used to order the
> returned rows is still name 'id'


Good spotting! This fix is in the CVS and will be part of 0.8.4.
--
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



 
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Kirk Haines
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Posts: n/a
 
      06-09-2004
On Thu, 10 Jun 2004 04:30:27 +0900, David Heinemeier Hansson wrote

> Oh, this was referring to database transactions. Before, if you
> attempted to create a database transaction block for MyISAM tables,
> it would bork. Now it'll just ignore MySQL's complaints.
>
> The object transactions are separate from this and require that you
> name the objects going into that with the transaction call.


Ah! I follow you.

Was the decision to do it that way mainly to make it easy to just use
Transaction::Simple?

To my mind, if one wants to perform a transaction, it is safer to just have
everything that could be under the transaction under it. I look at it as a
parallel structure sort of thing. At the database level, a transaction is a
transaction for everything. So, at the object level, when a transaction is
entered, all of the objects are part of that transaction. There is also a
database transaction that is running below that at the same time. That way,
if one wants to directly write some SQL and bypass the normal object methods
of doing things, that SQL still executes within the context of the
transaction.

Is there a benefit to being able to pick and choose which objects are part
of the transaction and which are not that I am being too dense to see right
now? What are your thoughts?


Thanks,

Kirk



 
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David Heinemeier Hansson
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Posts: n/a
 
      06-09-2004
> Is there a benefit to being able to pick and choose which objects are
> part
> of the transaction and which are not that I am being too dense to see
> right
> now? What are your thoughts?


It could very well be that you're interested in the data that cause the
invalidation. So let's imagine that name = "David" will cause the
transaction to fail. I'd really to be able to access that value
afterwards. This is how the validations stuff in AR works as well. So
attempt to validate the data, if it fails, you can present the wrong
data for correction.

That and, personally, I don't really need the object transactions most
of the time. So it's nice to be able to turn it off by not specifically
requiring it. Transactions are not free of performance penalties (as
Austin mentioned).
--
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



 
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Kirk Haines
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Posts: n/a
 
      06-09-2004
On Thu, 10 Jun 2004 04:52:00 +0900, David Heinemeier Hansson wrote

> It could very well be that you're interested in the data that cause
> the invalidation. So let's imagine that name = "David" will cause
> the transaction to fail. I'd really to be able to access that value
> afterwards. This is how the validations stuff in AR works as well.
> So attempt to validate the data, if it fails, you can present the
> wrong data for correction.


If a transaction fails because of an exception, the exception can be caught
before the rollback, though. And if the rollback is an explicit one -- the
code decides that something is wrong and calls a rollback before any
exception is encountered -- then one can choose to do something with the
incorrect value before the rollback.

> That and, personally, I don't really need the object transactions
> most of the time. So it's nice to be able to turn it off by not
> specifically requiring it. Transactions are not free of performance
> penalties (as Austin mentioned).


I'm finding that object transactions are handy for performance and/or for
validation. If one is making a lot of changes to an ORM object, but it's
only the final value that is really of long term interest, wrapping
everything in a transaction means that only the final value makes its way to
the db. Or, if the values in the object are subject to user input and
manipulation, you probably don't want to write that data out to the database
until it has been validated. So while I wasn't sure when I implemented
them, I'm finding myself using them all over the place.

Performance...Hmmm. There is an overhead to transactions. That's one of
the things I have been working on minimizing, myself. Can't really counter-
argue you there.

Here's another question -- is there any way to do transactions over
object/row insertions with ActiveRecord? i.e. start a transaction; do
something that creates a new object/row; incur and exception or decide that
there is a problem and rollback; table/objects are unchanged. No? Maybe?
Yes? My gut sense is that you can't because you do have to specify the
objects to have transactions applied to them when entering the transaction
block, and if the object doesn't exist then, you can't do that. However, am
I wrong?


Thanks again. This is interesting stuff.


Kirk Haines



 
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John W. Long
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Posts: n/a
 
      06-10-2004
David Heinemeier Hansson wrote:
> * Create new Active Record objects by setting the attributes through a
> block. Like this:
>
> person = Person.new do |p|
> p.name = 'Freddy'
> p.age = 19
> end


Why is the better than:

person = Person.new(
'name' => 'David',
'age' => 12
)

--
John



 
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