h2. Active Resource Basics

This guide should provide you with all you need to get started managing the connection between business objects and RESTful web services. It implements a way to map web-based resources to local objects with CRUD semantics.

endprologue.

WARNING. This Guide is based on Rails 3.0. Some of the code shown here will not work in earlier versions of Rails.

h3. Introduction

Active Resource allows you to connect with RESTful web services. So, in Rails, Resource classes inherited from +ActiveResource::Base+ and live in  +app/models+.

h3. 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:

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

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

h3. Reading and Writing Data

Active Resource make request over HTTP using a standard JSON format. It mirrors the RESTful routing built into Action Controller but will also work with any other REST service that properly implements the protocol.

h4. Read

Read requests use the GET method and expect the JSON form of whatever resource/resources is/are being requested.

<ruby>
# Find a person with id = 1
person = Person.find(1)
# Check if a person exists with id = 1
Person.exists?(1)  # => true
# Get all resources of Person class
Person.all
</ruby>

h4. Create

Creating a new resource submits the JSON form of the resource as the body of the request with HTTP POST method and parse the response into Active Resource object.

<ruby>
person = Person.create(:name => 'Vishnu')
person.id  # => 1
</ruby>

h4. Update

To update an existing resource, 'save' method is used. This method make a HTTP PUT request in JSON format.

<ruby>
person = Person.find(1)
person.name = 'Atrai'
person.save
</ruby>

h4. Delete

'destroy' method makes a HTTP DELETE request for an existing resource in JSON format to delete that resource.

<ruby>
person = Person.find(1)
person.destroy
</ruby>

h3. Validations

Module to support validation and errors with Active Resource objects. The module overrides Base#save to rescue ActiveResource::ResourceInvalid exceptions and parse the errors returned in the web service response. The module also adds an errors collection that mimics the interface of the errors provided by ActiveModel::Errors.

h4. Validating client side resources by overriding validation methods in base class

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

   protected

   def validate
     errors.add("last", "has invalid characters") unless last =~ /[a-zA-Z]*/
   end
end
</ruby>

h4. Validating client side resources

Consider a Person resource on the server requiring both a first_name and a last_name with a validates_presence_of :first_name, :last_name declaration in the model:

<ruby>
person = Person.new(:first_name => "Jim", :last_name => "")
person.save                   # => false (server returns an HTTP 422 status code and errors)
person.valid?                 # => false
person.errors.empty?          # => false
person.errors.count           # => 1
person.errors.full_messages   # => ["Last name can't be empty"]
person.errors[:last_name]  # => ["can't be empty"]
person.last_name = "Halpert"
person.save                   # => true (and person is now saved to the remote service)
</ruby>

h4. Public instance methods

ActiveResource::Validations have three public instance methods

h5. errors()

This will return errors object that holds all information about attribute error messages

h5. save_with_validation(options=nil)

This validates the resource with any local validations written in base class and then it will try to POST if there are no errors.

h5. valid?

Runs all the local validations and will return true if no errors.