How to "pretty" format JSON output in Ruby on Rails

Use the pretty_generate() function, built into later versions of JSON. For example:

require 'json'my_object = { :array => [1, 2, 3, { :sample => "hash"} ], :foo => "bar" }puts JSON.pretty_generate(my_object)

Which gets you:

{  "array": [    1,    2,    3,    {      "sample": "hash"    }  ],  "foo": "bar"}

The <pre> tag in HTML, used with JSON.pretty_generate, will render the JSON pretty in your view. I was so happy when my illustrious boss showed me this:

<% if @data.present? %>   <pre><%= JSON.pretty_generate(@data) %></pre><% end %>

Thanks to Rack Middleware and Rails 3 you can output pretty JSON for every request without changing any controller of your app. I have written such middleware snippet and I get nicely printed JSON in browser and curl output.

class PrettyJsonResponse  def initialize(app)    @app = app  end  def call(env)    status, headers, response = @app.call(env)    if headers["Content-Type"] =~ /^application\/json/      obj = JSON.parse(response.body)      pretty_str = JSON.pretty_unparse(obj)      response = [pretty_str]      headers["Content-Length"] = pretty_str.bytesize.to_s    end    [status, headers, response]  endend

The above code should be placed in app/middleware/pretty_json_response.rb of your Rails project.And the final step is to register the middleware in config/environments/development.rb:

config.middleware.use PrettyJsonResponse

I don't recommend to use it in production.rb. The JSON reparsing may degrade response time and throughput of your production app. Eventually extra logic such as 'X-Pretty-Json: true' header may be introduced to trigger formatting for manual curl requests on demand.

(Tested with Rails 3.2.8-5.0.0, Ruby 1.9.3-2.2.0, Linux)