Optimistic locking in Grape

config.ru

$LOAD_PATH.unshift(File.join(File.dirname(__FILE__)))

require 'grape'
require 'my_api'

use Rack::ConditionalGet
use Rack::ETag
run MyAPI

Gemfile

source 'http://rubygems.org'

gem 'grape'

my_api.rb

require 'optimistic_locking'

class MyAPI < Grape::API

  helpers OptimisticLocking

  resource "widgets" do
    params do
      requires :id
    end
    route_param :id do
      params do
        requires :name
      end
      put do
        widget = Widget.find(params[:id])
        optimistic_lock!(widget.to_json)
        widget.update(name: params[:name])
        widget.to_json
      end
    end
  end

end

optimistic_locking.rb

require 'digest'

# When a GET request is returned +Rack::ETaag+ will return the +ETag+
# header to the client, the client then needs to return the etag in the
# +If-None-Match+ header to modify the resource.
module OptimisticLocking
  def optimistic_lock!(body)
    digest = Digest::MD5.new
    digest << body
    if request.env["HTTP_IF_NONE_MATCH"] != %("#{digest}")
      error!('Object is stale, If-None-Match header does not match object digest.', 409)
    end
  end
end