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

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RABL is a ruby templating system for Rails and Padrino that takes a new approach to generating JSON and other formats. Rather than using the ActiveRecord 'to_json', I generally find myself wanting a more expressive and powerful system for generating my APIs. This is especially important when the json representation is complex or doesn't match the exact schema defined in the database itself.

There were a few things in particular I wanted to do easily:

  • Create arbitrary nodes named based on combining data in an object
  • Include nodes only if a certain condition is met
  • Pass arguments to methods and store the result as a child node
  • Partial templates and inheritance to reduce code duplication
  • Easily renaming attributes from their name in the model
  • Simple way to append attributes from a child into the parent

The list goes on. Anyone who has used the 'to_json' approach used in ActiveRecord for generating a json response has felt the pain of the extremely restrictive system. RABL is a general templating system created to solve all of those problems. When I created RABL, I wanted a simple, expressive DRY ruby DSL for defining JSON responses for my APIs.


Install as a gem:

gem install rabl

or add to your Gemfile:

# Gemfile
gem 'rabl'

and run bundle install to install the dependency.

If you are using Rails 2.X or Padrino, RABL works without configuration. With Sinatra, or any other tilt-based framework, simply register:


and RABL will be initialized and ready for use.


Object Assignment

To declare the data object for use in the template:

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

or a collection works:

 object @users

and this will be used as the default data object for the rendering.


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`
# { bar : 5 }
attribute :foo => :bar

or even multiple aliased attributes:

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

Child Nodes

You can also add child nodes from an arbitrary source:

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

or simply use existing model associations:

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

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

Use glue to add additional attributes to the parent object.

Custom Nodes

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

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

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


Often you need to access sub-objects in order to construct your own custom nodes for more complex associations. You can get access to the rabl representation of another object with:

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

or an object associated to the parent model:

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

You can use this method to construct arbitrarily complex nodes for your APIs.


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"

code :can_drink do |m|
  m.age > 21

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"

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


  • I am sloppy and once again failed to unit test this. Don't use it in production until I do obviously.
  • No support for Rails 3

Authors and Contributors


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

Thanks again for all of these great projects.

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