Automatically encode Rack output with JSON when Content-Type is application/json - ruby

Question

I've got a modular Sinatra app and I'd like to encode the output as JSON when the content-type dictates. At the moment I'm doing so manually in my routes:
get 'someroute' do
# content-type is actually set with a before filter
# included only for clarity
content_type 'application/json', :charset => 'utf-8'
# .. #
{:success=>true}.to_json
end
I'd prefer it to look like this:
get 'someroute' do
content_type 'application/json', :charset => 'utf-8'
# .. #
{:success=>true}
end
And I'd like to use Rack middleware to do the encoding if it detects the appropriate content-type.
I've been trying to get the following to work, but to no avail (the content-length gets borked - returns content-length of original content and not JSON encoded content):
require 'init'
module Rack
class JSON
def initialize app
#app = app
end
def call env
#status, #headers, #body = #app.call env
if json_response?
#body = #body.to_json
end
[#status, #headers, #body]
end
def json_response?
#headers['Content-Type'] == 'application/json'
end
end
end
use Rack::JSON
MyApp.set :run, false
MyApp.set :environment, ENV['RACK_ENV'].to_sym
run MyApp
Any pointers to get me back on track?

Answer 1

You’ve got everything right but one thing: Rack expects the body to be an object that responds to each but is not a string. Just put your body inside an array.
It’s probably not necessary, but if you want to set the content length by hand, just add it to the headers.
if json_response?
#body = [#body.to_json]
#headers["Content-Length"] = #body[0].size.to_s
end