ActiveResource::Base is the main class for mapping RESTful resources as models in a Rails application.

For an outline of what Active Resource is capable of, see link:files/vendor/rails/activeresource/README.html.

Automated mapping

Active Resource objects represent your RESTful resources as manipulatable Ruby objects. To map resources to Ruby objects, Active Resource only needs a class name that corresponds to the resource name (e.g., the class Person maps to the resources people, very similarly to Active Record) and a site value, which holds the URI of the resources.

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

Now the Person class is mapped to RESTful resources located at api.people.com:3000/people/, and you can now use Active Resource’s lifecycles methods to manipulate resources.

Lifecycle methods

Active Resource exposes methods for creating, finding, updating, and deleting resources from REST web services.

ryan = Person.new(:first => 'Ryan', :last => 'Daigle')
ryan.save  #=> true
ryan.id  #=> 2
Person.exists?(ryan.id)  #=> true
ryan.exists?  #=> true

ryan = Person.find(1)
# => Resource holding our newly created Person object

ryan.first = 'Rizzle'
ryan.save  #=> true

ryan.destroy  #=> true

As you can see, these are very similar to Active Record’s lifecycle methods for database records. You can read more about each of these methods in their respective documentation.

Custom REST methods

Since simple CRUD/lifecycle methods can’t accomplish every task, Active Resource also supports defining your own custom REST methods. To invoke them, Active Resource provides the get, post, put and delete methods where you can specify a custom REST method name to invoke.

# POST to the custom 'register' REST method, i.e. POST /people/new/register.xml.
Person.new(:name => 'Ryan').post(:register)
# => { :id => 1, :name => 'Ryan', :position => 'Clerk' }

# PUT an update by invoking the 'promote' REST method, i.e. PUT /people/1/promote.xml?position=Manager.
Person.find(1).put(:promote, :position => 'Manager')
# => { :id => 1, :name => 'Ryan', :position => 'Manager' }

# GET all the positions available, i.e. GET /people/positions.xml.
Person.get(:positions)
# => [{:name => 'Manager'}, {:name => 'Clerk'}]

# DELETE to 'fire' a person, i.e. DELETE /people/1/fire.xml.
Person.find(1).delete(:fire)

For more information on using custom REST methods, see the ActiveResource::CustomMethods documentation.

Validations

You can validate resources client side by overriding validation methods in the base class.

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

See the ActiveResource::Validations documentation for more information.

Authentication

Many REST APIs will require authentication, usually in the form of basic HTTP authentication. Authentication can be specified by putting the credentials in the site variable of the Active Resource class you need to authenticate.

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

For obvious security reasons, it is probably best if such services are available over HTTPS.

Errors & Validation

Error handling and validation is handled in much the same manner as you’re used to seeing in Active Record. Both the response code in the HTTP response and the body of the response are used to indicate that an error occurred.

Resource errors

When a GET is requested for a resource that does not exist, the HTTP 404 (Resource Not Found) response code will be returned from the server which will raise an ActiveResource::ResourceNotFound exception.

# GET http://api.people.com:3000/people/999.xml
ryan = Person.find(999) # => Raises ActiveResource::ResourceNotFound
# => Response = 404

404 is just one of the HTTP error response codes that ActiveResource will handle with its own exception. The following HTTP response codes will also result in these exceptions:

These custom exceptions allow you to deal with resource errors more naturally and with more precision rather than returning a general HTTP error. For example:

begin
  ryan = Person.find(my_id)
rescue ActiveResource::ResourceNotFound
  redirect_to :action => 'not_found'
rescue ActiveResource::ResourceConflict, ActiveResource::ResourceInvalid
  redirect_to :action => 'new'
end

Validation errors

Active Resource supports validations on resources and will return errors if any these validations fail (e.g., "First name can not be blank" and so on). These types of errors are denoted in the response by a response code of 422 and an XML representation of the validation errors. The save operation will then fail (with a false return value) and the validation errors can be accessed on the resource in question.

ryan = Person.find(1)
ryan.first #=> ''
ryan.save  #=> false

# When
# PUT http://api.people.com:3000/people/1.xml
# is requested with invalid values, the response is:
#
# Response (422):
# <errors type="array"><error>First cannot be empty</error></errors>
#

ryan.errors.invalid?(:first)  #=> true
ryan.errors.full_messages  #=> ['First cannot be empty']

Learn more about Active Resource’s validation features in the ActiveResource::Validations documentation.