= Active Resource

Active Resource (ARes) connects business objects and Representational State Transfer (REST)
web services.  It implements object-relational mapping for REST web services to provide transparent
proxying capabilities between a client (ActiveResource) and a RESTful service (which is provided by Simply RESTful routing
in ActionController::Resources).

== Philosophy

Active Resource attempts to provide a coherent wrapper object-relational mapping for REST
web services. It follows the same philosophy as Active Record, in that one of its prime aims
is to reduce the amount of code needed to map to these resources.  This is made possible
by relying on a number of code- and protocol-based conventions that make it easy for Active Resource
to infer complex relations and structures.  These conventions are outlined in detail in the documentation
for ActiveResource::Base.

== Overview

Model classes are mapped to remote REST resources by Active Resource much the same way Active Record maps model classes to database
tables.  When a request is made to a remote resource, a REST XML request is generated, transmitted, and the result
received and serialized into a usable Ruby object.

=== Configuration and Usage

Putting Active Resource to use is very similar to Active Record.  It's as simple as creating a model class
that inherits from ActiveResource::Base and providing a <tt>site</tt> class variable to it:

   class Person < ActiveResource::Base
     self.site = "http://api.people.com:3000/"
   end

Now the Person class is REST enabled and can invoke REST services very similarly to how Active Record invokes
lifecycle methods that operate against a persistent store.

   # Find a person with id = 1
   ryan = Person.find(1)
   Person.exists?(1)  # => true

As you can see, the methods are quite similar to Active Record's methods for dealing with database
records.  But rather than dealing directly with a database record, you're dealing with HTTP resources (which may or may not be database records).

==== Protocol

Active Resource is built on a standard XML format for requesting and submitting resources over HTTP.  It mirrors the RESTful routing
built into Action Controller but will also work with any other REST service that properly implements the protocol.
REST uses HTTP, but unlike "typical" web applications, it makes use of all the verbs available in the HTTP specification:

* GET requests are used for finding and retrieving resources.
* POST requests are used to create new resources.
* PUT requests are used to update existing resources.
* DELETE requests are used to delete resources.

For more information on how this protocol works with Active Resource, see the ActiveResource::Base documentation;
for more general information on REST web services, see the article here[http://en.wikipedia.org/wiki/Representational_State_Transfer].

==== Find

Find requests use the GET method and expect the XML form of whatever resource/resources is/are being requested.  So,
for a request for a single element, the XML of that item is expected in response:

   # Expects a response of
   #
   # <person><id type="integer">1</id><attribute1>value1</attribute1><attribute2>..</attribute2></person>
   #
   # for GET http://api.people.com:3000/people/1.xml
   #
   ryan = Person.find(1)

The XML document that is received is used to build a new object of type Person, with each
XML element becoming an attribute on the object.

   ryan.is_a? Person  # => true
   ryan.attribute1  # => 'value1'

Any complex element (one that contains other elements) becomes its own object:

   # With this response:
   #
   # <person><id>1</id><attribute1>value1</attribute1><complex><attribute2>value2</attribute2></complex></person>
   #
   # for GET http://api.people.com:3000/people/1.xml
   #
   ryan = Person.find(1)
   ryan.complex  # => <Person::Complex::xxxxx>
   ryan.complex.attribute2  # => 'value2'

Collections can also be requested in a similar fashion

   # Expects a response of
   #
   # <people type="array">
   #  <person><id type="integer">1</id><first>Ryan</first></person>
   #  <person><id type="integer">2</id><first>Jim</first></person>
   # </people>
   #
   # for GET http://api.people.com:3000/people.xml
   #
   people = Person.find(:all)
   people.first  # => <Person::xxx 'first' => 'Ryan' ...>
   people.last  # => <Person::xxx 'first' => 'Jim' ...>

==== Create

Creating a new resource submits the XML form of the resource as the body of the request and expects
a 'Location' header in the response with the RESTful URL location of the newly created resource.  The
id of the newly created resource is parsed out of the Location response header and automatically set
as the id of the ARes object.

  # <person><first>Ryan</first></person>
  #
  # is submitted as the body on
  #
  # POST http://api.people.com:3000/people.xml
  #
  # when save is called on a new Person object.  An empty response is
  # is expected with a 'Location' header value:
  #
  # Response (201): Location: http://api.people.com:3000/people/2
  #
  ryan = Person.new(:first => 'Ryan')
  ryan.new?  # => true
  ryan.save  # => true
  ryan.new?  # => false
  ryan.id    # => 2

==== Update

'save' is also used to update an existing resource - and follows the same protocol as creating a resource
with the exception that no response headers are needed - just an empty response when the update on the
server side was successful.

  # <person><first>Ryan</first></person>
  #
  # is submitted as the body on
  #
  # PUT http://api.people.com:3000/people/1.xml
  #
  # when save is called on an existing Person object.  An empty response is
  # is expected with code (204)
  #
  ryan = Person.find(1)
  ryan.first # => 'Ryan'
  ryan.first = 'Rizzle'
  ryan.save  # => true

==== Delete

Destruction of a resource can be invoked as a class and instance method of the resource.

  # A request is made to
  #
  # DELETE http://api.people.com:3000/people/1.xml
  #
  # for both of these forms.  An empty response with
  # is expected with response code (200)
  #
  ryan = Person.find(1)
  ryan.destroy  # => true
  ryan.exists?  # => false
  Person.delete(2)  # => true
  Person.exists?(2) # => false


You can find more usage information in the ActiveResource::Base documentation.