mongo.rb

require 'fiber'

module EventMachine
  module Deferrable
    def wait
      fiber = Fiber.current
      self.errback {|err| p err }
      self.callback {|cb| fiber.resume(cb) }
      Fiber.yield
    end
  end
end

module EM::Mongo
  class FiberDatabase < Database
    def authenticate(*args)
      super(*args).wait
    end
  end

  class FiberConnection < Connection
    def initialize(*args)
      fiber = Fiber.current
      super(*args)
      # establish connection before returning
      EM.next_tick { fiber.resume }
      Fiber.yield
    end
  end

  class FiberCollection < EM::Mongo::Collection
    %w{
      safe_insert
      safe_update
      find_and_modify
      map_reduce
      distinct
      group
      drop_index
      insert_documents
      safe_send
      index_name
    }.each do |name|
      class_eval <<-EOF
        def #{name}(*args)
          super(*args).wait
        end
      EOF
    end

    alias :afind :find

    def find(*args)
      super(*args).to_a.wait
    end

    def find_one(selector = {}, opts = {})
      opts[:limit] = 1
      find(selector, opts).first
    end

    alias :first :find_one

    def count
      afind().count.wait
    end
  end

  # Overwrite originals
  EM::Mongo::Database   = FiberDatabase
  EM::Mongo::Connection = FiberConnection
  EM::Mongo::Collection = FiberCollection
end

I saw your tweet. Nice work. How do you plug this into MongoMapper? Does this API map 1:1 to the mongo/plucky API?

def wait
  fib = Fiber.current
  callback do |res|
    fib.resume(res)
  end
  errback do |err|
    fib.resume(err)
  end
  res = Fiber.yield
  raise res if Exception === res
  res
end