Representable

Representable maps Ruby objects to documents and back.

In other words: Take an object and decorate 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 or populate 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.

Installation

The representable gem runs with all Ruby versions >= 1.8.7.

gem 'representable'

Example

What if we're writing an API for music - songs, albums, bands.

class Song < OpenStruct
end

song = Song.new(title: "Fallout", track: 1)

Defining Representations

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=.

Rendering

Mixing in the representer into the object adds a rendering method.

song.extend(SongRepresenter).to_json
#=> {"title":"Fallout","track":1}

Parsing

It also adds support for parsing.

song = Song.new.extend(SongRepresenter).from_json(%{ {"title":"Roxanne"} })
#=> #<Song title="Roxanne", track=nil>

Extend vs. Decorator

If you don't want representer modules to be mixed into your objects (using #extend) you can use the Decorator strategy described below. Decorating instead of extending was introduced in 1.4.

Aliasing

If your property name doesn't match the name in the document, use the :as option.

module SongRepresenter
  include Representable::JSON

  property :title, as: :name
  property :track
end

song.to_json #=> {"name":"Fallout","track":1}

Wrapping

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.

Collections

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: ["Stewart Copeland", "Sting"]).
  extend(SongRepresenter).to_json

#=> {"title":"Fallout","composers":["Stewart Copeland","Sting"]}

And again, this works both ways - in addition to the title it extracts the composers from the document, too.

Nesting

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":["Stewart 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"]>]>

Parse Strategies

When parsing collections (also applies to single propertys), representable usually iterates the incoming list and creates a new object per array item.

Parse strategies let you do that manually.

collection :songs, :parse_strategy => lambda { |fragment, i, options|
  songs << song = Song.new
  song
}

The above code will add a new Song per incoming item. Each instance will still be extended and populated with attributes (note that you can change that as well).

This gives you all the freedom you need for your nested parsing.

Syncing Objects

Usually, representable creates a new nested object when parsing. If you want to update an existing object, use the parse_strategy: :sync option.

module AlbumRepresenter
  include Representable::JSON

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

When parsing an album, it will now call from_json on the existing songs in the collection.

album = Album.find(1)
album.songs.first #=> #<Song:0x999 title: "Panama">

Note that the album already contains a song instance.

album.extend(AlbumRepresenter).
  from_json('{songs: [{title: "Eruption"}]}')

album.songs.first #=> #<Song:0x999 title: "Eruption">

Now, representable didn't create a new Song instance but updated the existing, resulting in renaming the song.

Find-or-Create For Incoming Objects

Representable comes with another strategy called :find_or_instantiate which allows creating a property or collection from the incoming document.

Consider the following incoming hash.

{"songs" => [{"id" => 1, "title" => "American Paradox"}, {"title" => "Uncoil"}}

And this representer setup.

collection :songs, class: Song, parse_strategy: :find_or_instantiate

In album.from_hash(..), representable will try to call Song.find(1) for the first songs collection element and Song.new for the second (as it doesn't has any id), resulting in an array of two Song instances, the first an existing, the second a new object.

Note: the various parsing strategies are a collection of "best practices" people find useful. Such a strategy is basically just a set of configuration options, mainly utilizing the :instance option.

Check out the ParsingStrategy module to write your own strategy. If you find it useful, please commit it to the core library (with tests).

The current state of the :find_or_instantiate strategy is subject to change.

Inline Representers

If you don't want to maintain two separate modules when nesting representations you can define the SongRepresenter inline.

module AlbumRepresenter
  include Representable::JSON

  property :name

  collection :songs, class: Song do
    property :title
    property :track
    collection :composers
  end

This works both for representer modules and decorators.

An inline representer is just a Ruby module. You can include other representer modules. This is handy when having a base representer that needs to be extended in the inline block.

module AlbumRepresenter
  include Representable::JSON

  property :hit do
    include SongRepresenter

    property :numbers_sold
  end

Document Nesting

Not always does the structure of the desired document map to your objects. The ::nested method allows you to structure properties in a separate section while still mapping the properties to the outer object.

Imagine the following document.

{"title": "Roxanne",
 "details":
   {"track": 3,
    "length": "4:10"}
}

However, both track and length are properties of the song object <Song#0x999 title: "Roxanne", track: 3 ...>, there is no such concept as details in the Song class. Representable gives you ::nested to achieve this.

class SongRepresenter < Representable::Decorator
  include Representable::JSON

  property :title

  nested :details do
    property :track
    property :length
  end
end

Just use an inline representer or the extend: option to define nested properties. Accessors for nested properties will still be called on the outer object (here, song). And as always, this works both ways for rendering and parsing.

Note that ::nested internally is implemented using Decorator. When adding methods inside the nested block, make sure to use represented (self will point to the decorator instance).

Decorator vs. Extend

People who dislike :extend go use the Decorator strategy!

class SongRepresentation < Representable::Decorator
  include Representable::JSON

  property :title
  property :track
end

The Decorator constructor requires the represented object.

SongRepresentation.new(song).to_json

This will leave the song instance untouched as the decorator just uses public accessors to represent the hit.

In compositions you need to specify the decorators for the nested items using the :decorator option where you'd normally use :extend.

class AlbumRepresentation < Representable::Decorator
  include Representable::JSON

  collection :songs, :class => Song, :decorator => SongRepresentation
end

Helpers In Decorators

In module representers you can add methods for properties.

module SongRepresenter
  property :title

  def title
    @name
  end

That works as the method is mixed into the represented object. When adding a helper method to a decorator, representable will still invoke accessors on the represented instance - unless you tell it the scope.

class SongRepresenter < Representable::Decorator
  property :title, exec_context: :decorator

  def title
    represented.name
  end
end

This will call title getter and setter on the decorator instance, not on the represented object. You can still access the represented object in the decorator method using represented. BTW, in a module representer this option setting is ignored.

Possible values for this switch (formerly known as :decorator_scope) are :binding, :decorator and nil, which is the default setting where lambdas and methods are invoked in the represented context.

Or use :getter or :setter to dynamically add a method for the represented object.

class SongRepresenter < Representable::Decorator
  property :title, getter: lambda { |*| @name }

As always, the block is executed in the represented object's context.

Passing Options

You're free to pass an options hash into the rendering or parsing.

song.to_json(:append => "SOLD OUT!")

If you want to append the "SOLD OUT!" to the song's title when rendering, use the :getter option.

SongRepresenter
  include Representable::JSON

  property :title, :getter => lambda { |args| title + args[:append] }
end

Note that the block is executed in the represented model context which allows using accessors and instance variables.

The same works for parsing using the :setter method.

property :title, :setter => lambda { |val, args| self.title= val + args[:append] }

Here, the block retrieves two arguments: the parsed value and your user options.

You can also use the :getter option instead of writing a reader method. Even when you're not interested in user options you can still use this technique.

property :title, :getter => lambda { |*| @name }

This hash will also be available in the :if block, documented here and will be passed to nested objects.

Dynamic Options

Most of property's options are dynamic, meaning the can be either a static value, a lambda or a :symbol refering to an instance method to be called.

All user options are passed to the lambdas, e.g. when you call

song.to_hash(volume: 9)

the lambda invocation for :as would look like this.

property :name, as: lambda do |args|
  args #=> {:volume=>9}
end

Available Options

Here's a list of all dynamic options and their argument signature.

Option Arguments

The pass_options: true option instructs representable to pass a special Options instance into lambdas or methods. This is handy if you need access to the other stakeholder objects involved in representing objects.

property :title, pass_options: true, getter: lambda do |args|
  args #=> <#Options>
  args.binding # etc.
end

The Options instance exposes the following readers: #binding, #represented, #decorator and #user_options which is the hash you usually have as args.

Option-specific arguments (e.g. fragment, see here) are still prepended, making the Options object always the last argument.

XML Support

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: ["Stewart Copeland", "Sting"]).
     extend(SongRepresenter).to_xml #=>
<song>
    <title>Fallout</title>
    <composers>Stewart Copeland</composers>
    <composers>Sting</composers>
</song>

Using Helpers

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

Inheritance

To reuse existing representers use inheritance.

Inheritance works by includeing already defined representers.

module CoverSongRepresenter
  include Representable::JSON
  include SongRepresenter

  property :copyright
end

This results in a representer with the following properties.

property :title     # inherited from SongRepresenter.
property :copyright

With decorators, you - surprisingly - use class inheritance.

class HitRepresenter < SongRepresenter
  collection :airplays

Overriding Properties

You might want to override a particular property in an inheriting representer. Successively calling property(name) will override the former definition for name just as you know it from overriding methods.

module CoverSongRepresenter
  include Representable::JSON

  include SongRepresenter        # defines property :title
  property :title, as: :known_as # overrides that definition.
end

This behaviour was added in 1.7.

Partly Overriding Properties

If you wanna override only certain options of the property, use :inherit.

module SongRepresenter
  include Representable::JSON

  property :title, as: :known_as
end

You can now inherit properties but still override or add options.

module CoverSongRepresenter
  include Representable::JSON
  include SongRepresenter

  property :title, getter: lambda { Title.random }, inherit: true
end

Using the :inherit, this will result in a property having the following options.

property :title,
  as:     :known_as,    # inherited from SongRepresenter.
  getter: lambda { .. } # added in inheriting representer.

Inheritance With Inline Representers

Inheriting also works for inline representers.

module SongRepresenter
  include Representable::JSON

  property :title
  property :label do
    property :name
  end
end

You can now override or add properties with the inline representer.

module HitRepresenter
  include Representable::JSON
  include SongRepresenter

  property :label, inherit: true do
    property :country
  end
end

Results in a combined inline representer as it inherits.

property :label do
  property :name
  property :country
end

Naturally, :inherit can be used within the inline representer block.

Note that the following also works.

module HitRepresenter
  include Representable::JSON
  include SongRepresenter

  property :label, as: :company, inherit: true
end

This renames the property but still inherits all the inlined configuration.

Basically, :inherit copies the configuration from the parent property, then merges in your options from the inheriting representer. It exposes the same behaviour as super in Ruby - when using :inherit the property must exist in the parent representer.

Polymorphic Extend

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.

Polymorphic Object Creation

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

Hashes

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}}

Lonely Hashes

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.

Lonely Collections

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.

YAML Support

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: [Stewart Copeland, Sting]

More on XML

Mapping Tag Attributes

You can also map properties to tag attributes in representable. This works only for the top-level node, though (seen from the representer's perspective).

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.

Mapping Content

The same concept can also be applied to content. If you need to map a property to the top-level node's content, use the :content option. Again, top-level refers to the document fragment that maps to the representer.

module SongRepresenter
  include Representable::XML

  property :title, content: true
end

Song.new(title: "American Idle").to_xml
#=> <song>American Idle</song>

Wrapping Collections

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>

Namespaces

Support for namespaces are not yet implemented. However, if an incoming parsed document contains namespaces, you can automatically remove them.

module AlbumRepresenter
  include Representable::XML

  remove_namespaces!

Avoiding Modules

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. Use decorators instead.

More Options

Here's a quick overview about other available options for #property and its bro #collection.

Overriding Read And Write

This can be handy if a property needs to be compiled from several fragments. The lambda has access to the entire object document (either hash or Nokogiri node) and user options.

property :title, :writer => lambda { |doc, args| doc["title"] = title || original_title }

When using the :writer option it is up to you to add fragments to the doc - representable won't add anything for this property.

The same works for parsing using :reader.

property :title, :reader => lambda { |doc, args| self.title = doc["title"] || doc["name"] }

Read/Write Restrictions

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.

Filtering

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"}

Conditions

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.

As always, the block retrieves your options. Given this render call

song.to_json(minimum_track: 2)

your :if may process the options.

property :track, if: lambda { |opts| track > opts[:minimum_track] }

False and Nil Values

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

Overriding Serialize And Deserialize

When serializing, the default mechanics after preparing the object are to call object.to_hash.

Override this step with :serialize.

property :song, extend: SongRepresenter,
  serialize: lambda { |object, *args| Marshal.dump(object) }

Vice-versa, parsing allows the same.

property :song, extend: SongRepresenter,
  deserialize: lambda { |object, fragment, *args| Marshal.load(fragment) }

Coercion

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 Representable::Coercion

  property :title
  property :recorded_at, :type => DateTime, :default => "May 12th, 2012"
end

In a decorator it works alike.

module SongRepresenter < Representable::Decorator
  include Representable::JSON
  include Representable::Coercion

  property :recorded_at, :type => DateTime
end

Coercing values only happens when rendering or parsing a document. Representable does not create accessors in your model as virtus does.

Undocumented Features

(Please don't read this section!)

Custom Binding

If you need a special binding for a property you're free to create it using the :binding option.

property :title, :binding => lambda { |*args| JSON::TitleBinding.new(*args) }

Syncing Parsing

You can use the parsed document fragment directly as a representable instance by returning nil in :class.

property :song, :class => lambda { |*| nil }

This makes sense when your parsing looks like this.

hit.from_hash(song: <#Song ..>)

Representable will not attempt to create a Song instance for you but use the provided from the document.

Note that this is now the official option :parse_strategy.

Rendering And Parsing Without Extend

Sometimes you wanna skip the preparation step when rendering and parsing, for instance, when the object already exposes a #to_hash/#from_hash method.

class ParsingSong
  def from_hash(hash, *args)
    # do whatever

    self
  end

  def to_hash(*args)
    {}
  end
end

This would work with a representer as the following.

property :song, :class => ParsingSong, prepare: lambda { |object| object }

Instead of automatically extending/decorating the object, the :prepare lambda is run. It's up to you to prepare you object - or simply return it, as in the above example.

Skipping Rendering Or Parsing

You can skip to call to #to_hash/#from_hash on the prepared object by using :representable.

property :song, :representable => false

This will run the entire serialization/deserialization without calling the actual representing method on the object.

Extremely helpful if you wanna use representable as a data mapping tool with filtering, aliasing, etc., without the rendering and parsing part.

Returning Arbitrary Objects When Parsing

When representable parses the song attribute, it calls ParsingSong#from_hash. This method could return any object, which will then be assigned as the song property.

class ParsingSong
  def from_hash(hash, *args)
    [1,2,3,4]
  end
end

Album.extend(AlbumRepresenter).from_hash(..).song #=> [1,2,3,4]

This also works with :extend where the specified module overwrites the parsing method (e.g. #from_hash).

Decorator In Module

Inline representers defined in a module can be implemented as a decorator, thus wrapping the represented object without pollution.

property :song, use_decorator: true do
  property :title
end

This is an implementation detail most people shouldn't worry about.

Copyright

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.

Representable is released under the MIT License.