mongo_id_oddities.rb

require 'rubygems'
require 'mongo'

connection = Mongo::Connection.new
database = connection.db 'oddball'
collection = database.create_collection('foo', :capped => true, :size => 10 * 1024 * 1024)

docs = []
100.times {|n| docs << {'count' => n, 'time' => Time.now.utc.to_s}}

collection.insert docs


doc = collection.find_one
puts "class: #{doc['_id'].class.name}, id: #{doc['_id'].inspect}"
puts collection.find_one(doc['_id']).inspect