application.json.jbuilder

raw_content = JSON.parse(yield)
json.content raw_content

json.jsonp! params[:callback] if jsonp?

jbuilder_extension.rb

Mime::EXTENSION_LOOKUP['jsonp'] = Mime::Type.lookup_by_extension('json')

require 'jbuilder'

module Jsonp::Jbuilder
  extend ActiveSupport::Concern

  included do
    alias_method_chain :target!, :jsonp
  end

  def jsonp!(callback_name)
    @jsonp_callback = callback_name
  end

  def target_with_jsonp!
    json = target_without_jsonp!
    if @jsonp_callback
      "#{@jsonp_callback}(#{json})"
    else
      json
    end
  end
  
end

::Jbuilder.send :include, Jsonp::Jbuilder