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RABL (Ruby API Builder Language) is a ruby templating system for rendering resources in different format (JSON, XML, BSON, ...). You can find documentation here.

rabl-rails is faster and uses less memory than the standard rabl gem while letting you access the same features. There are some slight changes to do on your templates to get this gem to work but it should't take you more than 5 minutes.

rabl-rails only targets Rails 4.2+ application and is compatible with mri 2.2+, jRuby and rubinius.


Install as a gem :

gem install rabl-rails

or add directly to your Gemfile

gem 'rabl-rails'

And that's it !


Once you have installed rabl-rails, you can directly used RABL-rails templates to render your resources without changing anything to you controller. As example, assuming you have a Post model filled with blog posts, and a PostController that look like this :

class PostController < ApplicationController
  respond_to :html, :json, :xml

  def index
	 @posts = Post.order('created_at DESC')

You can create the following RABL-rails template to express the API output of @posts

# app/views/post/index.rabl
collection :@posts
attributes :id, :title, :subject
child(:user) { attributes :full_name }
node(:read) { |post| post.read_by?(@user) }

This would output the following JSON when visiting http://localhost:3000/posts.json

  "id" : 5, title: "...", subject: "...",
  "user" : { full_name : "..." },
  "read" : true

That's a basic overview but there is a lot more to see such as partials, inheritance or fragment caching.

How it works

As opposed to standard RABL gem, this gem separate compiling (a.k.a transforming a RABL-rails template into a Ruby hash) and the actual rendering of the object or collection. This allow to only compile the template once and only Ruby hashes.

The fact of compiling the template outside of any rendering context prevent us to use any instances variables (with the exception of node) in the template because they are rendering objects. So instead, you'll have to use symbols of these variables.For example, to render the collection @posts inside your PostController, you need to use :@posts inside of the template.

The only places where you can actually used instance variables are into Proc (or lambda) or into custom node (because they are treated as Proc).

# We reference the @posts varibles that will be used at rendering time
collection :@posts

# Here you can use directly the instance variable because it
# will be evaluated when rendering the object
node(:read) { |post| post.read_by?(@user) }

The same rule applies for view helpers such as current_user

After the template is compiled into a hash, Rabl-rails will use a renderer to do the actual output. Actually, only JSON and XML formats are supported.


RablRails works out of the box, with default options and fastest engine available (oj, libxml). But depending on your needs, you might want to change that or how your output looks like. You can set global configuration in your application:

# config/initializers/rabl_rails.rb
RablRails.configure do |config|
  # These are the default
  # config.cache_templates = true
  # config.include_json_root = true
  # config.json_engine = ::Oj
  # config.xml_options = { :dasherize => true, :skip_types => false }
  # config.enable_jsonp_callbacks = false
  # config.replace_nil_values_with_empty_strings = false
  # config.replace_empty_string_values_with_nil = false
  # config.exclude_nil_values = false
  # config.non_collection_classes =['Struct'])


Data declaration

To declare data to use in the template, you can use either object or collection with the symbol name or your data.

# app/views/users/show.json.rabl
object :@user

# app/views/users/index.json.rabl
collection :@users

You can specify root label for the collection using hash or :root option

collection :@posts, root: :articles
#is equivalent to
collection :@posts => :articles

# => { "articles" : [{...}, {...}] }

There are rares cases when the template doesn't map directly to any object. In these cases, you can set data to false or skip data declaration altogether.

object false
node(:some_count) { |_| @user.posts.count }
child(:@user) { attribute :name }

If you use gem like decent_exposure or focused_controller, you can use your variable directly without the leading @

object :object_exposed

You can even skip data declaration at all. If you used respond_with, rabl-rails will render the data you passed to it. As there is no name, you can set a root via the root macro. This allow you to use your template without caring about variables passed to it.

# in controller

# in rabl-rails template
root :article
attribute :title

Attributes / Methods

Basic usage is to declared attributes to include in the response. These can be database attributes or any instance method.

attributes :id, :title, :to_s

You can aliases these attributes in your response

attributes title: :foo, to_s: :bar
# => { "foo" : <title value>, "bar" : <to_s value> }

or show attributes only if a condition is true

attributes :published_at, :anchor, if: ->(post) { post.published? }

Child nodes

You can include informations from data associated with the parent model or arbitrary data. These informations can be grouped under a node or directly merged into current node.

For example if you have a Post model that belongs to a User

object :@post
child(user: :author) do
	attributes :name
# => { "post" : { "author" : { "name" : "John D." } } }

You can also use arbitrary data source with child nodes

child(:@users) do
	attributes :id, :name

If you want to merge directly into current node, you can use the glue keywork

attribute :title
glue(:user) do
  attributes :name => :author_name
# => { "post" : { "title" : "Foo", "author_name" : "John D." } }

Custom nodes

You can create custom node in your response, based on the result of a given block

object :@user
node(:full_name) { |u| u.first_name + " " + u.last_name }
# => { "user" : { "full_name" : "John Doe" } }

or with an assigned constant

const(:api_version, API::VERSION)
const(:locale, 'fr_FR')

You can add condition on your custom nodes (if the condition is evaluated to false, the node will not be included).

node(:email, if: ->(u) { u.valid_email? }) do |u|

Nodes are evaluated at the rendering time, so you can use any instance variables or view helpers inside them

node(:url) { |post| post_url(post) }

If you want to include directly the result into the current node, use the merge keyword (result returned from the block should be a hash)

object :@user
merge { |u| { name: u.first_name + " " + u.last_name } }
# => { "user" : { "name" : "John Doe" } }

Custom nodes are really usefull to create flexible representations of your resources.

Extends & Partials

Often objects have a basic representation that is shared accross different views and enriched according to it. To avoid code redundancy you can extend your template from any other RABL template.

# app/views/users/base.json.rabl
attributes :id, :name

# app/views/users/private.json.rabl
attributes :super_secret_attribute

extends 'users/base'
# or using partial instead of extends
# merge { |u| partial('users/base', object: u) }

You can also extends template in child nodes using partial option (this is the same as using extends in the child block)

collection @posts
attribute :title
child(:user, partial: 'users/base')

Partials can also be used inside custom nodes. When using partial this way, you MUST declare the object associated to the partial

node(:location) do |user|
	{ city:, address: partial('users/address', object: m.address) }

When used within node, partials can take locals variables that can be accessed in the included template.

# base.json.rabl
node(:credit_card, if: ->(u) { locals[:display_credit_card] }) do |user|

# user.json.rabl
merge { |u| partial('users/base', object: u, locals: { display_credit_card: true }) }


Rabl allow you to define easily your templates, even with hierarchy of 2 or 3 levels. Let's suppose your have a thread model that has many posts and that each post has many comments. We can display a full thread in a few lines

object :@thread
attribute :caption
child :posts do
  attribute :title
	child :comments do
		extends 'comments/base'

Other features

And more in the WIKI


Benchmarks have been made using this application, with rabl 0.13.1 and rabl-rails 0.5.0

Overall, rabl-rails is 10% faster and use 10% less memory, but these numbers skyrockets to 50% when using extends with collection of objects.

You can see full tests on test application repository.

Authors and contributors

Want to add another format to Rabl-rails ? Checkout JSON renderer for reference Want to make another change ? Just fork and contribute, any help is very much appreciated. If you found a bug, you can report it via the Github issues.

Original idea

  • RABL Standart RABL gem. I used it a lot but I needed to improve my API response time, and since most of the time was spent in view rendering, I decided to implement a faster rabl gem.


Copyright © 2012-2017 Christopher Cocchi-Perrier. See MIT-LICENSE for details.

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