app.rb

require 'sinatra/base'


require 'dm-core'
require 'dm-migrations'
require 'dm-serializer/to_json'

module BestAppEver
  class App < Sinatra::Base    
    configure :development do
      DataMapper::setup(:default, "sqlite3://#{Dir.pwd}/db/best_app_ever_development.db")
      DataMapper::Logger.new(STDOUT, :debug)
    end

    DataMapper::Model.raise_on_save_failure = true
    DataMapper.finalize 
    DataMapper.auto_upgrade!

    get '/users' do
      users = User.all

      users.to_json
    end  
  end
end

user.rb

require 'dm-core'
require 'dm-validations'
require 'dm-serializer/to_json'

class User
  include DataMapper::Resource
  
  property :id,    Serial
  property :uid,   String
  property :token, String
  
  has n, :discussions
  
  def as_json(*a)
    {
      id:    id.to_s,
      uid:   uid,
      token: token
    }
  end     
end