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RABL

RABL (Ruby API Builder Language) is a Rails and Padrino ruby templating system for generating JSON, XML, MessagePack, PList and BSON. When using the ActiveRecord 'to_json' method, I tend to quickly find myself wanting a more expressive and powerful solution for generating APIs. This is especially frustrating when the JSON representation is complex or doesn't match the exact schema defined in the database.

I wanted a simple and flexible system for generating my APIs. In particular, I wanted to easily:

  • Create arbitrary nodes named based on combining data in an object
  • Pass arguments to methods and store the result as a child node
  • Render partial templates and inherit to reduce code duplication
  • Rename or alias attributes to change the name from the model
  • Append attributes from a child into a parent node
  • Include nodes only if a certain condition has been met

Anyone who has tried the 'to_json' method used in ActiveRecord for generating a JSON response has felt the pain of this restrictive approach. RABL is a general templating system created to solve these problems in an entirely new way.

Installation

Install RABL as a gem:

gem install rabl

or add to your Gemfile:

# Gemfile
gem 'rabl'
# Also add either `json` or `yajl-ruby` as the JSON parser
gem 'yajl-ruby'

and run bundle install to install the dependency.

If you are using Rails 2.X, Rails 3.X or Padrino, RABL works without configuration.

With Sinatra, or any other tilt-based framework, simply register:

Rabl.register!

and RABL will be initialized and ready for use. For usage with Sinatra, check out the Sinatra Usage guide.

Note: Users have reported a few rendering issues with Rails 3.2. The template handler probably needs a patch to properly support Rails 3.2. Hopefully I can get to it soon but patches are welcome.

Overview

You can use RABL to generate JSON and XML based APIs from any ruby object. With RABL, the data typically is derived primarily from models (ORM-agnostic) and the representation of the API output is described within a view template using a simple ruby DSL. This allows you to keep your data separated from the JSON or XML you wish to output.

Once you have installed RABL (explained above), you can construct a RABL view template and then render the template from your Sinatra, Padrino or Rails applications from the controller (or route) very easily. Using Padrino as an example, assuming you have a Post model filled with blog posts, you can render an API representation (both JSON and XML) by creating a route:

# app/app.rb
get "/posts", :provides => [:json, :xml] do
  @user = current_user
  @posts = Post.order("id DESC")
  render "posts/index"
end

Then we can create the following RABL template to express the API output of @posts:

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

Which would output the following JSON or XML when visiting http://localhost:3000/posts.json

[{  post :
  {
    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, custom nodes, etc. Read the full details of RABL below.

Configuration

RABL is intended to require little to no configuration to get working. This is the case in most scenarios, but depending on your needs you may want to set the following global configurations in your application (this block is completely optional):

# config/initializers/rabl_init.rb
Rabl.configure do |config|
  # Commented as these are defaults
  # config.cache_sources = false
  # config.json_engine = nil # Any multi\_json engines
  # config.msgpack_engine = nil # Defaults to ::MessagePack
  # config.bson_engine = nil # Defaults to ::BSON
  # config.plist_engine = nil # Defaults to ::Plist::Emit
  # config.include_json_root = true
  # config.include_msgpack_root = true
  # config.include_bson_root = true
  # config.include_plist_root = true
  # config.include_xml_root  = false
  # config.enable_json_callbacks = false
  # config.xml_options = { :dasherize  => true, :skip_types => false }
end

Each option specifies behavior related to RABL's output. If include_json_root is disabled that removes the root node for each child in the output, and enable_json_callbacks enables support for 'jsonp' style callback output if the incoming request has a 'callback' parameter.

If cache_sources is set to true, template lookups will be cached for improved performance. The cache can be reset manually by running Rabl.reset_source_cache! within your application.

Note that the json_engine option uses the multi_json intelligent engine defaults so in most cases you don't need to configure this directly. If you wish to use yajl as the primary JSON encoding engine simply add that to your Gemfile:

# Gemfile
gem 'yajl-ruby', :require => "yajl"

and RABL will automatically start using that engine for encoding your JSON responses!

Message Pack

Rabl also includes optional support for Message Pack serialization format using the msgpack gem. To enable, include the msgpack gem in your project's Gemfile. Then use Rabl as normal with the msgpack format (akin to json and xml formats).

# Gemfile
gem 'msgpack', '~> 0.4.5'

One can additionally use a custom Message Pack implementation by setting the Rabl msgpack_engine configuration attribute. This custom message pack engine must conform to the MessagePack#pack method signature.

class CustomEncodeEngine
  def self.pack string
    # Custom Encoding by your own engine.
  end
end

Rabl.configure do |config|
  config.msgpack_engine = CustomEncodeEngine
end

NOTE: Attempting to render the msgpack format without either including the msgpack gem or setting a msgpack_engine will cause an exception to be raised.

BSON

Rabl also includes optional support for BSON serialization format using the bson gem. To enable, include the bson gem in your project's Gemfile. Then use Rabl as normal with the bson format (akin to json and xml formats).

# Gemfile
gem 'bson', '~> 1.5.2'

To use it with Rails, also register the bson mime type format:

# config/initializers/mime_types.rb
Mime::Type.register "application/bson", :bson

One can additionally use a custom BSON implementation by setting the Rabl bson_engine configuration attribute. This custom BSON engine must conform to the BSON#serialize method signature.

class CustomEncodeEngine
  def self.serialize string
    # Custom Encoding by your own engine.
  end
end

Rabl.configure do |config|
  config.bson_engine = CustomEncodeEngine
end

NOTE: Attempting to render the bson format without either including the bson gem or setting a bson_engine will cause an exception to be raised.

Plist

Rabl also includes optional support for Plist serialization format using the plist gem. To enable, include the plist gem in your project's Gemfile. Then use Rabl as normal with the plist format (akin to other formats).

# Gemfile
gem 'plist'

There is also an option for a custom Plist implementation by setting the Rabl plist_engine configuration attribute.

class CustomEncodeEngine
  def self.dump string
    # Custom Encoding by your own engine.
  end
end

Rabl.configure do |config|
  config.plist_engine = CustomEncodeEngine
end

NOTE: Attempting to render the plist format without either including the plist gem or setting a plist_engine will cause an exception to be raised.

Usage

Object Assignment

To declare the data object for use in the template:

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

or specify an alias for the object:

object @user => :person
# => { "person" : { ... } }

or pass a collection of objects:

collection @users
# => [ { "user" : { ... } } ]

or specify a root node label for the collection:

collection @users => :people
# => { "people" : [ { "person" : { ... } } ] }

or even specify both the child and root labels for a collection:

collection @users, :root => "people", :object_root => "user"
# => { "people" : [ { "user" : { ... } } ] }

and this will be used as the default data for the rendering, or disable the object root explicitly:

collection @users, :root => "people", :object_root => false
# => { "people" : [ { ... }, { ... } ] }

There can also be odd cases where the root-level of the response doesn't map directly to any object:

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

In those cases, object can be assigned to 'false' and nodes can be constructed free-form.

Attributes

Basic usage of the templater to define a few simple attributes for the response:

# app/views/users/show.json.rabl
attributes :id, :foo, :bar

or use with aliased attributes:

# Take the value of model attribute `foo` and name the node `bar`
attribute :foo => :bar
# => { bar : 5 }

or even multiple aliased attributes:

attributes :bar => :baz, :dog => :animal
# => # { baz : <bar value>, animal : <dog value> }

Child Nodes

Often a response requires including nested information from data associated with the parent model:

child :address do
  attributes :street, :city, :zip, :state
end

You can also add child nodes from an arbitrary data source:

child @posts => :foobar do
  attributes :id, :title
end

or use model associations with an alias:

# Renders all the 'posts' association
# from the model into a node called 'foobar'
child :posts => :foobar do
  attributes :id, :title
end

Gluing Attributes

You can also append child attributes back to the root node:

# Appends post_id and post_name to parent json object
glue @post do
  attributes :id => :post_id, :name => :post_name
end

Use glue to add additional attributes to the parent object.

Custom Nodes

This will generate a json response based on the result of the node block:

# app/views/users/show.json.rabl
node :full_name do |u|
  u.first_name + " " + u.last_name
end

or a custom node that exists only if a condition is true:

# m is the object being rendered, also supports :unless
node(:foo, :if => lambda { |m| m.has_foo? }) do |m|
  m.foo
end

or don't pass a name and have the node block merged into the response:

node do |u|
  { :full_name => u.first_name + " " + u.last_name }
  # => { full_name : "Bob Johnson" }
end

You can use custom nodes like these to create flexible representations of a value utilizing all the data from the model.

Partials

Often you need to access other data objects in order to construct custom nodes in more complex associations. You can get access to the rabl representation of another data object by rendering a RABL partial:

node :location do
  { :city => @city, :address => partial("users/address", :object => @address) }
end

or event access an object associated with the parent model:

node :location do |m|
  { :city => m.city, :address => partial("users/address", :object => m.address) }
end

You can use this method to construct arbitrarily complex nodes for your APIs. Note that you need to have RABL templates defined for each of the objects you wish to construct representations for in this manner.

Inheritance

Another common issue of many template builders is unnecessary code redundancy. Typically many representations of an object across multiple endpoints share common attributes or nodes. The nodes for a 'post' object are probably the same or similar in most references throughout the various endpoints.

RABL has the ability to extend other "base" rabl templates and additional attributes:

# app/views/users/advanced.json.rabl
extends "users/base" # another RABL template in "app/views/users/base.json.rabl"

node :can_drink do |m|
  m.age > 21
end

You can also extend other rabl templates while constructing child nodes to reduce duplication:

# app/views/users/show.json.rabl
child @address do
  extends "address/item"
end

Using partials and inheritance can significantly reduce code duplication in your templates.

Template Scope

In RABL, you have access to everything you need to build an API response. Each RABL template has full access to the controllers instance variables as well as all view helpers and routing urls.

# app/some/template.rabl
object @post
# Access instance variables
child(@user => :user) { ... }
# or Rails helpers
node(:formatted_body) { |post| simple_format(post.body) }

There should be no problem fetching the appropriate data to construct a response.

Deep Nesting

In APIs, you can often need to construct 2nd or 3rd level nodes. Let's suppose we have a 'quiz' model that has many 'questions' and then each question has many 'answers'. We can display this hierarchy in RABL quite easily:

# app/views/quizzes/show.json.rabl
object @quiz
attribute :title
child :questions do
  attribute :caption
  child :answers do
    # Use inheritance to reduce duplication
    extends "answers/item"
  end
end

This will display the quiz object with nested questions and answers as you would expect with a quiz node, and embedded questions and answers. Note that RABL can be nested arbitrarily deep within child nodes to allow for these representations to be defined.

Resources

There are many resources available relating to RABL including the RABL Wiki, and many tutorials and guides detailed below.

Advanced Usage

Links to resources for advanced usage:

Please add your own usages and let me know so we can add them here! Also be sure to check out the RABL Wiki for other usages.

Tutorials

Tutorials can always be helpful when first getting started:

Let me know if there's any other useful resources not listed here.

Related Libraries

There are several libraries that either complement or extend the functionality of RABL:

  • grape-rabl - Allows rabl templates to be used with grape
  • gon - Exposes your Rails variables in JS with RABL support integrated.

Let me know if there's any other related libraries not listed here.

Troubleshooting

Examples

See the examples directory.

Issues

Check out the Issues tab for a full list:

  • Better Tilt template support (precompiling templates)
  • Benchmarks and performance optimizations

Continuous Integration

Continuous Integration status

CI is hosted by travis-ci.org.

Authors and Contributors

Thanks to Miso for allowing me to create this for our applications and release this project!

and many more contributors listed in the CHANGELOG.

Want to contribute support for another format? Check out the patches for msgpack support, plist support and BSON support for reference.

Please fork and contribute, any help in making this project better is appreciated!

Inspirations

There are a few excellent libraries that helped inspire RABL and they are listed below:

Thanks again for all of these great projects.

Copyright

Copyright © 2011-2012 Nathan Esquenazi. See MIT-LICENSE for details.

About

General ruby templating with json, bson, xml, plist and msgpack support

engineering.gomiso.com/2011/06/27/building-a-platform-api-on-rails/

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