Resource-Oriented Architectures in Ruby.

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Roar is a framework for parsing and rendering REST documents. Nothing more.

Representers let you define your API document structure and semantics. They allow both rendering representations from your models and parsing documents to update your Ruby objects. The bi-directional nature of representers make them interesting for both server and client usage.

Roar comes with built-in JSON, JSON-HAL, JSON-API and XML support. Its highly modular architecture provides features like coercion, hypermedia, HTTP transport, client caching and more.

Roar is completely framework-agnostic and loves being used in web kits like Rails, Webmachine, Sinatra, Padrino, etc. If you use Rails, consider roar-rails for an enjoyable integration.


Roar is just a thin layer on top of the representable gem. While Roar gives you a DSL and behaviour for creating hypermedia APIs, representable implements all the mapping functionality.

If in need for a feature, make sure to check the representable API docs first.


The roar gem runs with all Ruby versions >= 1.9.3.

ruby gem 'roar'


Roar does not bundle dependencies for JSON and XML.

If you want to use JSON, add the following to your Gemfile:

ruby gem 'multi_json'

If you want to use XML, add the following to your Gemfile:

ruby gem 'nokogiri'

Defining Representers

Let’s see how representers work. They’re fun to use.

```ruby require ‘roar/json’

module SongRepresenter include Roar::JSON

property :title end ```

API documents are defined using a representer module or decorator class. You can define plain attributes using the ::property method.

Now let’s assume we’d have Song which is an ActiveRecord class. Please note that Roar is not limited to ActiveRecord. In fact, it doesn’t really care whether it’s representing ActiveRecord, Datamapper or just an OpenStruct instance.

ruby class Song < ActiveRecord::Base end


To render a document, you apply the representer to your model.

```ruby song = “Fate”) song.extend(SongRepresenter)

song.to_json #=> “title”:”Fate” ```

Here, the representer is injected into the actual model and gives us a new #to_json method.


The cool thing about representers is: they can be used for rendering and parsing. See how easy updating your model from a document is.

```ruby song = song.extend(SongRepresenter)


song.title #=> Linoleum ```

Again, #from_json comes from the representer and just updates the known properties.

Unknown attributes in the parsed document are simply ignored, making half-baked solutions like strong_parameters redundant.


Many people dislike #extend due to eventual performance issue or object pollution. If you’re one of those, just go with a decorator representer. They almost work identical to the module approach we just discovered.

```ruby require ‘roar/decorator’ require ‘roar/json’

class SongRepresenter < Roar::Decorator include Roar::JSON

property :title end ``` In place of a module you use a class, the DSL inside is the same you already know.

```ruby song = “Medicine Balls”) #=> Balls” ```

Here, the song objects gets wrapped (or “decorated”) by the decorator. It is treated as immutuable - Roar won’t mix in any behaviour.

Note that decorators and representer modules have identical features. You can parse, render, nest, go nuts with both of them.

However, in this README we’ll use modules to illustrate this framework.


Roar (or rather representable) also allows mapping collections in documents.

```ruby module SongRepresenter include Roar::JSON

property :title collection :composers end ```

Where ::property knows how to handle plain attributes, ::collection does lists.

```ruby song = “Roxanne”, composers: [“Sting”, “Stu Copeland”]) song.extend(SongRepresenter)

song.to_json #=> Copeland”] ```

And, yes, this also works for parsing: from_json will create and populate the array of the composers attribute.


Now what if we need to tackle with collections of Songs? We need to implement an Album class.

ruby class Album < ActiveRecord::Base has_many :songs end

Another representer to represent.

```ruby module AlbumRepresenter include Roar::JSON

property :title collection :songs, extend: SongRepresenter, class: Song end ```

Both ::property and ::collection accept options for nesting representers into representers.

The extend: option tells Roar which representer to use for the nested objects (here, the array items of the album.songs field). When parsing a document class: defines the nested object type.

Consider the following object setup.

ruby album = "True North") album.songs << "The Island") album.songs << => "Changing Tide")

You apply the AlbumRepresenter and you get a nested document.

```ruby album.extend(AlbumRepresenter)

album.to_json #=> North”,”songs”:[{“title”:”The Island”,Tide”]} ```

This works vice-versa.

```ruby album = album.extend(AlbumRepresenter)


puts album.songs[1] #=> # ```

The nesting of two representers can map composed object as you find them in many many APIs.

In case you’re after virtual nesting, where a nested block in your document still maps to the same outer object, check out the ::nested method.

Inline Representer

Sometimes you don’t wanna create two separate representers - although it makes them reusable across your app. Use inline representers if you’re not intending this.

```ruby module AlbumRepresenter include Roar::JSON

property :title

collection :songs, class: Song do property :title end end ```

This will give you the same rendering and parsing behaviour as in the previous example with just one module.

Syncing Objects

Usually, when parsing, nested objects are created from scratch. If you want nested objects to be updated instead of being newly created, use parse_strategy:.

```ruby module AlbumRepresenter include Roar::JSON

property :title

collection :songs, extend: SongRepresenter, parse_strategy: :sync end ```

This will advise Roar to update existing songs.

```ruby album.songs[0].object_id #=> 81431220

album.from_json(‘North”,”songs”:[{“title”:”Secret Society”,Tide”]}’)

album.songs[0].title #=> Secret Society album.songs[0].object_id #=> 81431220 ``` Roar didn’t create a new Song instance but updated its attributes, only.

We’re currently working on better strategies to easily implement POST and PUT semantics in your APIs without having to worry about the nitty-gritties.


Roar provides coercion with the virtus gem.

```ruby require ‘roar/coercion’ require ‘roar/json’

module SongRepresenter include Roar::JSON include Roar::Coercion

property :title property :released_at, type: DateTime end ```

The :type option allows to set a virtus-compatible type.

```ruby song = song.extend(SongRepresenter)


song.released_at #=> 1981-03-31T00:00:00+00:00 ```

More Features

Roar/representable gives you many more mapping features like renaming attributes, wrapping, passing options, etc. See the representable documentation for a detailed explanation.


Roar comes with built-in support for embedding and processing hypermedia in your documents.

```ruby module SongRepresenter include Roar::JSON include Roar::Hypermedia

property :title

link :self do “http://songs/#title” end end ```

The Hypermedia feature allows declaring links using the ::link method. In the block, you have access to the represented model. When using representer modules, the block is executed in the model’s context.

However, when using decorators, the context is the decorator instance, allowing you to access additional data. Use represented to retrieve model data.

ruby class SongRepresenter < Roar::Decorator # .. link :self do "http://songs/#{represented.title}" end end

This will render links into your representation.

ruby song.extend(SongRepresenter) song.to_json #=> {"title":"Roxanne","links":[{"rel":"self","href":"http://songs/Roxanne"}]}

Per default, links are pushed into the hash using the links key. Link blocks are executed in represented context, allowing you to call any instance method of your model (here, we call #title).

Also, note that roar-rails allows using URL helpers in link blocks.

Passing Options

Sometimes you need more data in the link block. Data that’s not available from the represented model.

```ruby module SongRepresenter include Roar::JSON

property :title

link :self do |opts| “http://#opts[:base_url]songs/#title” end end ```

Pass this data to the rendering method.

ruby song.to_json(base_url: "localhost:3001/")

Any options passed to #to_json will be available as block arguments in the link blocks.

Specify Decorator

If you have a property that is a separate class or model, you can specify a decorator for that property. Suppose there is a separate Artist model for an album. When the album is eagerly loaded, the artist model could be represented along with it.

```ruby class ArtistRepresenter < Roar::Decorator property :name end

class AlbumRepresenter < Roar::Decorator # .. property :artist, decorator: ArtistRepresenter end ```

For using modules, use extend: ArtistRepresenter.

Consuming Hypermedia

Since we defined hypermedia attributes in the representer we can also consume this hypermedia when we parse documents.

```ruby song.from_json(‘“title”:”Roxanne”,”links”:[{“rel”:”self”,”href”:”http://songs/Roxanne”]}’)

song.links[:self].href #=> “http://songs/Roxanne” ```

Reading link attributes works by using #links[] on the consuming instance.

This allows an easy way to discover hypermedia and build navigational logic on top.

Media Formats

While Roar comes with a built-in hypermedia format, there’s official media types that are widely recognized. Roar currently supports HAL and JSON API.

Simply by including a module you make your representer understand the media type. This makes it easy to change formats during evaluation.


The HAL format is a simple media type that defines embedded resources and hypermedia.

```ruby require ‘roar/json/hal’

module SongRepresenter include Roar::JSON::HAL

property :title

link :self do “http://songs/#title” end end ```

Documentation for HAL can be found in the API docs.

Make sure you understand the different contexts for links when using decorators.


Including the Roar::JSON::HAL module adds some more DSL methods to your module. It still allows using ::link but treats them slightly different.

ruby song.to_json #=> {"title":"Roxanne","_links":{"self":{"href":"http://songs/Roxanne"}}}

According to the HAL specification, links are now key with their rel attribute under the _links key.

Parsing works like-wise: Roar will use the same setters as before but it knows how to read HAL.


Nested, or embedded, resources can be defined using the :embedded option.

```ruby module AlbumRepresenter include Roar::JSON::HAL

property :title

collection :songs, class: Song, embedded: true do property :title end end ```

To embed a resource, you can use an inline representer or use :extend to specify the representer name.

```ruby album.to_json

=> North”,”_embedded”:{“songs”:[{“title”:”The Island”,Tide”]}}


HAL keys nested resources under the _embedded key and then by their type.

All HAL features in Roar are discussed in the API docs, including array links.


Roar also supports JSON API via the Roar JSON API gem.

Client-Side Support

Being a bi-directional mapper that does rendering and parsing, Roar representers are perfectly suitable for use in clients, too. In many projects, representers are shared as gems between server and client.

Consider the following shared representer.

```ruby module SongRepresenter include Roar::JSON include Roar::Hypermedia

property :title property :id

link :self do “http://songs/#title” end end ```

In a client where you don’t have access to the database it is common to use OpenStruct classes as domain objects.

```ruby require ‘roar/client’ require ‘roar/json’

class Song < OpenStruct include Roar::JSON include SongRepresenter include Roar::Client end ```

HTTP Support

The Client module mixes all necessary methods into the client class, e.g. it provides HTTP support

```ruby song = “Roxanne”) “http://localhost:4567/songs”, as: “application/json”) #=> 42 ```

What happens here?

  • You’re responsible for initializing the client object with attributes. This can happen with in the constructor or using accessors.
  • post will use the included SongRepresenter to compile the document using #to_json.
  • The document gets POSTed to the passed URL.
  • If a document is returned, it is deserialized and the client’s attributes are updated.

This is a very simple but efficient mechanism for working with representations in a client application.

Roar works with all HTTP request types, check out GET.

```ruby song = song.get(uri: “http://localhost:4567/songs/1”, as: “application/json”)

song.title #=> “Roxanne” song.links[:self].href #=> http://localhost/songs/1 ```

As GET is not supposed to send any data, you can use #get on an empty object to populate it with the server data.


Roar supports SSL connections - they are automatically detected via the protocol.

ruby song.get(uri: "https://localhost:4567/songs/1")

Basic Authentication

The HTTP verbs allow you to specify credentials for HTTP basic auth.

```ruby song.get(uri: “http://localhost:4567/songs/1”, basic_auth: [“username”, “secret_password”])


Client SSL certificates

(Only currently supported with Net:Http)

```ruby song.get(uri: “http://localhost:4567/songs/1”, pem_file: “/path/to/client/cert.pem”, ssl_verify_mode: OpenSSL::SSL::VERIFY_PEER)


Note: ssl_verify_mode is not required and will default to OpenSSL::SSL::VERIFY_PEER)

Request customization

All verbs yield the request object before the request is sent, allowing to modify it. It is a Net::HTTP::Request instance (unless you use Faraday).

ruby song.get(uri: "http://localhost:4567/songs/1") do |req| req.add_field("Cookie", "Yumyum") end

Error handling

In case of a non-2xx response status, #get and friends raise a Roar::Transport::Error exception. The original response can be accessed as follows.

ruby song.get(uri: "http://localhost/songs1") # not-existing. rescue Roar::Transport::Error => exception exception.response.code #=> 404


Roar also comes with XML support.

```ruby module SongRepresenter include Roar::XML include Roar::Hypermedia

property :title property :id

link :self do “http://songs/#title” end end ```

Include the Roar::XML engine and get bi-directional XML for your objects.

```ruby song = “Roxanne”, id: 42) song.extend(XML::SongRepresenter)

song.to_xml ```

Note that you now use #to_xml and #from_xml.


Roxanne 42


Please consult the representable XML documentation for all its great features.


Questions? Need help? Free 1st Level Support on ! We also have a mailing list, yiha!


Roar is released under the MIT License.