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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 that takes a different approach for generating JSON and other formats. Rather than using the ActiveRecord 'to_json', I generally find myself wanting to use a more expressive and flexible system for generating my Public APIs. This is especially true when I the json doesn't match to the exact schema defined in the database.

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

  • Create arbitrary nodes named based on combining data in the object
  • Include nodes only if a condition is met
  • Pass arguments to methods and store the result as a node
  • Include partial templates to reduce code duplication
  • Easily rename attributes from their name in the model

This general templating system solves all of those problems.


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 out of the box. With Sinatra, or any other tilt-based framework, simply register:


and RABL will be initialized.


Object Assignment

To declare the data object to 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:

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

or with aliased attributes:

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

or multiple aliased attributes:

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

Child Nodes

You can also add children nodes from an arbitrary object:

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

or use existing model associations:

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

Glued Attributes

You can also append attributes to the root node:

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 with the attributes specified. You can also include arbitrary code:

# 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 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 hash representation of another object:

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 these to construct arbitrarily complex nodes for APIs.


Another common limitation of many json builders is code redundancy. Typically every representation of an object across 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 in constructing 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.
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