A simple rack middleware that allows you to post JSON body data. We are using the YAJL json library for ruby. Feel free to substitute your own.

gistfile1.rb

require 'yajl'

module Rack

  # A Rack middleware for parsing POST/PUT body data when Content-Type is
  # <tt>application/json</tt>.
  #
  class JsonPostBody

    # Constants
    #
    POST_BODY  = 'rack.input'.freeze
    FORM_HASH  = 'rack.request.form_hash'.freeze
    FORM_INPUT = 'rack.request.form_input'.freeze

    def initialize(app)
      @app = app
    end

    def call(env)
      if env['CONTENT_TYPE'] =~ %r{application/json}i
        body = env[POST_BODY].read

        # make sure we have a hash before parsing
        if body =~ %r/^\s*\{/
          env.update(FORM_HASH => Yajl::Parser.parse(body), FORM_INPUT => env[POST_BODY])
        end
      end
      @app.call(env)
    end

  end
end

See also: https://github.com/rack/rack-contrib/blob/master/lib/rack/contrib/post_body_content_type_parser.rb

what if you want don't want to just parse, but to replace the post body entirely?