Representable maps ruby objects to documents and back.
In other words: Take an object and extend it with a representer module. This will allow you to render a JSON, XML or YAML document from that object. But that's only half of it! You can also use representers to parse a document and create an object.
Representable is helpful for all kind of rendering and parsing workflows. However, it is mostly useful in API code. Are you planning to write a real REST API with representable? Then check out the roar gem first, save work and time and make the world a better place instead.
The representable gem is almost dependency-free. Almost.
gem 'representable'
What if we're writing an API for music - songs, albums, bands.
class Song < OpenStruct
end
song = Song.new(title: "Fallout", track: 1)
Representations are defined using representer modules.
require 'representable/json'
module SongRepresenter
include Representable::JSON
property :title
property :track
end
In the representer the #property method allows declaring represented attributes of the object. All the representer requires for rendering are readers on the represented object, e.g. #title and #track. When parsing, it will call setters - in our example, that'd be #title= and #track=.
Mixing in the representer into the object adds a rendering method.
song.extend(SongRepresenter).to_json
#=> {"title":"Fallout","track":1}
It also adds support for parsing.
song = Song.new.extend(SongRepresenter).from_json(%{ {"title":"Roxanne"} })
#=> #<Song title="Roxanne", track=nil>
Let the representer know if you want wrapping.
module SongRepresenter
include Representable::JSON
self.representation_wrap= :hit
property :title
property :track
end
This will add a container for rendering and consuming.
song.extend(SongRepresenter).to_json
#=> {"hit":{"title":"Fallout","track":1}}
Setting self.representation_wrap = true will advice representable to figure out the wrap itself by inspecting the represented object class.
Let's add a list of composers to the song representation.
module SongRepresenter
include Representable::JSON
property :title
property :track
collection :composers
end
Surprisingly, #collection lets us define lists of objects to represent.
Song.new(title: "Fallout", composers: ["Steward Copeland", "Sting"]).
extend(SongRepresenter).to_json
#=> {"title":"Fallout","composers":["Steward Copeland","Sting"]}
And again, this works both ways - in addition to the title it extracts the composers from the document, too.
Representers can also manage compositions. Why not use an album that contains a list of songs?
class Album < OpenStruct
end
album = Album.new(name: "The Police", songs: [song, Song.new(title: "Synchronicity")])
Here comes the representer that defines the composition.
module AlbumRepresenter
include Representable::JSON
property :name
collection :songs, extend: SongRepresenter, class: Song
end
Note that nesting works with both plain #property and #collection.
When rendering, the :extend module is used to extend the attribute(s) with the correct representer module.
album.extend(AlbumRepresenter).to_json
#=> {"name":"The Police","songs":[{"title":"Fallout","composers":["Steward Copeland","Sting"]},{"title":"Synchronicity","composers":[]}]}
Parsing a documents needs both :extend and the :class option as the parser requires knowledge what kind of object to create from the nested composition.
Album.new.extend(AlbumRepresenter).
from_json(%{{"name":"Offspring","songs":[{"title":"Genocide"},{"title":"Nitro","composers":["Offspring"]}]}})
#=> #<Album name="Offspring", songs=[#<Song title="Genocide">, #<Song title="Nitro", composers=["Offspring"]>]>
While representable does a great job with JSON, it also features support for XML, YAML and pure ruby hashes.
require 'representable/xml'
module SongRepresenter
include Representable::XML
property :title
property :track
collection :composers
end
For XML we just include the Representable::XML module.
Song.new(title: "Fallout", composers: ["Steward Copeland", "Sting"]).
extend(SongRepresenter).to_xml
#=> <song>
<title>Fallout</title>
<composers>Steward Copeland</composers>
<composers>Sting</composers>
</song>
Sometimes it's useful to override accessors to customize output or parsing.
module AlbumRepresenter
include Representable::JSON
property :name
collection :songs
def name
super.upcase
end
end
Album.new(:name => "The Police").
extend(AlbumRepresenter).to_json
#=> {"name":"THE POLICE","songs":[]}
Note how the representer allows calling super in order to access the original attribute method of the represented object.
To change the parsing process override the setter.
def name=(value)
super(value.downcase)
end
To reuse existing representers you can inherit from those modules.
module CoverSongRepresenter
include Representable::JSON
include SongRepresenter
property :copyright
end
Inheritance works by includeing already defined representers.
Song.new(:title => "Truth Hits Everybody", :copyright => "The Police").
extend(CoverSongRepresenter).to_json
#=> {"title":"Truth Hits Everybody","copyright":"The Police"}
Sometimes heterogenous collections of objects from different classes must be represented. Or you don't know which representer to use at compile-time and need to delay the computation until runtime. This is why :extend accepts a lambda, too.
Given we not only have songs, but also cover songs.
class CoverSong < Song
end
And a non-homogenous collection of songs.
songs = [ Song.new(title: "Weirdo", track: 5),
CoverSong.new(title: "Truth Hits Everybody", track: 6, copyright: "The Police")]
album = Album.new(name: "Incognito", songs: songs)
The CoverSong instances are to be represented by their very own CoverSongRepresenter defined above. We can't just use a static module in the :extend option, so go use a dynamic lambda!
module AlbumRepresenter
include Representable::JSON
property :name
collection :songs, :extend => lambda { |song| song.is_a?(CoverSong) ? CoverSongRepresenter : SongRepresenter }
end
Note that the lambda block is evaluated in the represented object context which allows to access helpers or whatever in the block. This works for single properties, too.
Rendering heterogenous collections usually implies that you also need to parse those. Luckily, :class also accepts a lambda.
module AlbumRepresenter
include Representable::JSON
property :name
collection :songs,
:extend => ...,
:class => lambda { |hsh| hsh.has_key?("copyright") ? CoverSong : Song }
end
The block for :class receives the currently parsed fragment. Here, this might be somthing like {"title"=>"Weirdo", "track"=>5}.
If this is not enough, you may override the entire object creation process using :instance.
module AlbumRepresenter
include Representable::JSON
property :name
collection :songs,
:extend => ...,
:instance => lambda { |hsh| hsh.has_key?("copyright") ? CoverSong.new : Song.new(original: true) }
end
As an addition to single properties and collections representable also offers to represent hash attributes.
module SongRepresenter
include Representable::JSON
property :title
hash :ratings
end
Song.new(title: "Bliss", ratings: {"Rolling Stone" => 4.9, "FryZine" => 4.5}).
extend(SongRepresenter).to_json
#=> {"title":"Bliss","ratings":{"Rolling Stone":4.9,"FryZine":4.5}}
Need to represent a bare hash without any container? Use the JSON::Hash representer (or XML::Hash).
require 'representable/json/hash'
module FavoriteSongsRepresenter
include Representable::JSON::Hash
end
{"Nick" => "Hyper Music", "El" => "Blown In The Wind"}.extend(FavoriteSongsRepresenter).to_json
#=> {"Nick":"Hyper Music","El":"Blown In The Wind"}
Works both ways. The values are configurable and might be self-representing objects in turn. Tell the Hash by using #values.
module FavoriteSongsRepresenter
include Representable::JSON::Hash
values extend: SongRepresenter, class: Song
end
{"Nick" => Song.new(title: "Hyper Music")}.extend(FavoriteSongsRepresenter).to_json
In XML, if you want to store hash attributes in tag attributes instead of dedicated nodes, use XML::AttributeHash.
Same goes with arrays.
require 'representable/json/collection'
module SongsRepresenter
include Representable::JSON::Collection
items extend: SongRepresenter, class: Song
end
The #items method lets you configure the contained entity representing here.
[Song.new(title: "Hyper Music"), Song.new(title: "Screenager")].extend(SongsRepresenter).to_json
#=> [{"title":"Hyper Music"},{"title":"Screenager"}]
Note that this also works for XML.
Representable also comes with a YAML representer.
module SongRepresenter
include Representable::YAML
property :title
property :track
collection :composers, :style => :flow
end
A nice feature is that #collection also accepts a :style option which helps having nicely formatted inline (or "flow") arrays in your YAML - if you want that!
song.extend(SongRepresenter).to_yaml
#=>
---
title: Fallout
composers: [Steward Copeland, Sting]
You can also map properties to tag attributes in representable.
module SongRepresenter
include Representable::XML
property :title, attribute: true
property :track, attribute: true
end
Song.new(title: "American Idle").to_xml
#=> <song title="American Idle" />
Naturally, this works for both ways.
It is sometimes unavoidable to wrap tag lists in a container tag.
module AlbumRepresenter
include Representable::XML
collection :songs, :as => :song, :wrap => :songs
end
Note that :wrap defines the container tag name.
Album.new.to_xml #=>
<album>
<songs>
<song>Laundry Basket</song>
<song>Two Kevins</song>
<song>Wright and Rong</song>
</songs>
</album>
There's been a rough discussion whether or not to use extend in Ruby. If you want to save that particular step when representing objects, define the representers right in your classes.
class Song < OpenStruct
include Representable::JSON
property :name
end
I do not recommend this approach as it bloats your domain classes with representation logic that is barely needed elsewhere.
Here's a quick overview about other available options for #property and its bro #collection.
Using the :readable and :writeable options access to properties can be restricted.
property :title, :readable => false
This will leave out the title property in the rendered document. Vice-versa, :writeable will skip the property when parsing and does not assign it.
Representable also allows you to skip and include properties using the :exclude and :include options passed directly to the respective method.
song.to_json(:include => :title)
#=> {"title":"Roxanne"}
You can also define conditions on properties using :if, making them being considered only when the block returns a true value.
module SongRepresenter
include Representable::JSON
property :title
property :track, if: lambda { track > 0 }
end
When rendering or parsing, the track property is considered only if track is valid. Note that the block is executed in instance context, giving you access to instance methods.
If your property name doesn't match the attribute name in the document, use the :as option.
module SongRepresenter
property :title
property :track, as: :track_number
end
song.to_json #=> {"title":"Superstars","track_number":1}
Since representable-1.2 false values are considered when parsing and rendering. That particularly means properties that used to be unset (i.e. nil) after parsing might be false now. Vice versa, false properties that weren't included in the rendered document will be visible now.
If you want nil values to be included when rendering, use the :render_nil option.
property :track, render_nil: true
If you fancy coercion when parsing a document you can use the Coercion module which uses virtus for type conversion.
Include virtus in your Gemfile, first. Be sure to include virtus 0.5.0 or greater.
gem 'virtus', ">= 0.5.0"
Use the :type option to specify the conversion target. Note that :default still works.
module SongRepresenter
include Representable::JSON
include Virtus
include Representable::Coercion
property :title
property :recorded_at, :type => DateTime, :default => "May 12th, 2012"
end
Representable started as a heavily simplified fork of the ROXML gem. Big thanks to Ben Woosley for his inspiring work.
- Copyright (c) 2011-2013 Nick Sutterer apotonick@gmail.com
- ROXML is Copyright (c) 2004-2009 Ben Woosley, Zak Mandhro and Anders Engstrom.