# ROAR

_Resource-Oriented Architectures in Ruby._

## Introduction

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 and XML support. Its highly modulare 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](https://github.com/apotonick/roar-rails) for an enjoyable integration.

## Representable

Roar is just a thin layer on top of the [representable](https://github.com/apotonick/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](https://github.com/apotonick/representable) first.


## Defining Representers

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

```ruby
require 'roar/representer/json'

module SongRepresenter
  include Roar::Representer::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
end
```

## Rendering

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

```ruby
song = Song.new(title: "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.

## Parsing

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.new
song.extend(SongRepresenter)

song.from_json('{"title":"Linoleum"}')

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.


## Decorator

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'

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

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

```ruby
song = Song.new(title: "Medicine Balls")

SongRepresenter.new(song).to_json #=> {"title":"Medicine 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.


## Collections

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

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

  property :title
  collection :composers
end
```

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

```ruby
song = Song.new(title: "Roxanne", composers: ["Sting", "Stu Copeland"])
song.extend(SongRepresenter)

song.to_json #=> {"title":"Roxanne","composers":["Sting","Stu Copeland"]}
```

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


## Nesting

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

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

Another representer to represent.

```ruby
module AlbumRepresenter
  include Roar::Representer::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 = Album.new(title: "True North")
album.songs << Song.new(title: "The Island")
album.songs << Song.new(:title => "Changing Tide")
```

You apply the `AlbumRepresenter` and you get a nested document.

```ruby
album.extend(AlbumRepresenter)

album.to_json #=> {"title":"True North","songs":[{"title":"The Island"},{"title":"Changing Tide"}]}
```

This works vice-versa.

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

album.from_json('{"title":"Indestructible","songs":[{"title":"Tropical London"},{"title":"Roadblock"}]}')

puts album.songs[1] #=> #<Song title="Roadblock">
```

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


## 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::Representer::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::Representer::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('{"title":"True North","songs":[{"title":"Secret Society"},{"title":"Changing 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](https://github.com/apotonick/roar/issues/85) better strategies to easily implement `POST` and `PUT` semantics in your APIs without having to worry about the nitty-gritties.


## Coercion

Roar provides coercion with the [virtus](https://github.com/solnic/virtus) gem.

```ruby
require 'roar/representer/feature/coercion'

module SongRepresenter
  include Roar::Representer::JSON
  include Roar::Representer::Feature::Coercion

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

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

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

song.from_json('{"released_at":"1981/03/31"}')

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


## More Features

Roar/representable gives you many more mapping features like [renaming attributes](https://github.com/apotonick/representable/#aliasing), [wrapping](https://github.com/apotonick/representable/#wrapping), [passing options](https://github.com/apotonick/representable/#passing-options), etc.


## Hypermedia

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

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

  property :title

  link :self do
    "http://songs/#{title}"
  end
end
```

The `Hypermedia` feature allows declaring links using the `::link` method.

```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](https://github.com/apotonick/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::Representer::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.


## 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 Collection+JSON. Support for Siren and JSON-API is planned when there's sponsors.

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

## HAL-JSON

The [HAL](http://stateless.co/hal_specification.html) format is a simple media type that defines embedded resources and hypermedia.

```ruby
require 'roar/representer/json/hal'

module SongRepresenter
  include Roar::Representer::JSON::HAL

  property :title

  link :self do
    "http://songs/#{title}"
  end
end
```

### Hypermedia

Including the `Roar::Representer::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.

### Nesting

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

```ruby
module AlbumRepresenter
  include Roar::Representer::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

#=> {"title":"True North","_embedded":{"songs":[{"title":"The Island"},{"title":"Changing 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](http://rdoc.info/github/apotonick/roar/Roar/Representer/JSON/HAL), including [array links](https://github.com/apotonick/roar/blob/master/lib/roar/representer/json/hal.rb#L176).


## Collection+JSON

The [Collection+JSON media format](http://amundsen.com/media-types/collection/) defines document format and semantics for requests. It is currently experimental as we're still exploring how we optimize the support with Roar. Let us know if you're using it.

```ruby
module SongRepresenter
  include Roar::Representer::JSON::CollectionJSON
  version "1.0"
  href { "http://localhost/songs/" }

  property :title

  items(:class => Song) do
    href { "//songs/#{title}" }

    property :title, :prompt => "Song title"

    link(:download) { "//songs/#{title}.mp3" }
  end

  template do
    property :title, :prompt => "Song title"
  end

  queries do
    link :search do
      {:href => "//search", :data => [{:name => "q", :value => ""}]}
    end
  end
end
```

It renders a document following the Collection+JSON specs.

```
#=> {"collection":{
  "template":{"data":[{"name":"title","value":null}]},
  "queries":[{"rel":"search","href":"//search","data":[{"name":"q","value":""}]}],
  "version":"1.0",
  "href":"http://localhost/songs/",
  "title":"Roxanne",
  "items":null}}
```

We have big plans with this media format, as the object model in Roar plays nicely with Collection+JSON's API semantics.


## 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::Representer::JSON
  include Roar::Representer::Feature::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/representer/feature/client'

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

### HTTP Support

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

```ruby
song = Song.new(title: "Roxanne")
song.post("http://localhost:4567/songs", "application/json")

song.id #=> 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 `POST`ed 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 = Client::Song.new
song.get("http://localhost:4567/songs/1", "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.


## XML

Roar also comes with XML support.

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

  property :title
  property :id

  link :self do
    "http://songs/#{title}"
  end
end
```

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

```ruby
song = Song.new(title: "Roxanne", id: 42)
song.extend(XML::SongRepresenter)

song.to_xml
```

Note that you now use `#to_xml` and `#from_xml`.

```xml
<song>
  <title>Roxanne</title>
  <id>42</id>
  <link rel="self" href="http://songs/Roxanne"/>
</song>
```

Please consult the [representable XML documentation](https://github.com/apotonick/representable/#more-on-xml) for all its great features.


## Support

Questions? Need help? Free 1st Level Support on irc.freenode.org#roar !
We also have a [mailing list](https://groups.google.com/forum/?fromgroups#!forum/roar-talk), yiha!

## License

Roar is released under the [MIT License](http://www.opensource.org/licenses/MIT).