Mongo allows objects as _id. _id is always indexed and Mongo knows how to index complex objects.

mind_blower_ruby19.rb

require 'pp'
require 'rubygems'
require 'mongo'

conn = Mongo::Connection.new
db = conn.db('test')
col = db['test']
col.remove

oid = BSON::ObjectId.new
oid2 = BSON::ObjectId.new

(1..3).each do |num|
  col.save(:_id => {:s => oid, :n => num})
end

(1..3).each do |num|
  col.save(:_id => {:s => oid2, :n => num})
end

pp col.find({
  :_id => {
    '$gte' => {:s => oid, :n => 2},
    '$lte' => {:s => oid, :n => 3},
  },
}).to_a

# Example output:
# [{"_id"=>{"s"=>BSON::ObjectId('4fc623684c114f24be000001'), "n"=>2}},
#  {"_id"=>{"s"=>BSON::ObjectId('4fc623684c114f24be000001'), "n"=>3}}]

mind_blower_ruby_18.rb

require 'pp'
require 'rubygems'
require 'mongo'

conn = Mongo::Connection.new
db = conn.db('test')
col = db['test']
col.remove

oid = BSON::ObjectId.new
oid2 = BSON::ObjectId.new

(1..3).each do |num|
  doc = BSON::OrderedHash.new
  doc[:s] = oid
  doc[:n] = num
  col.save(:_id => doc)
end

(1..3).each do |num|
  doc = BSON::OrderedHash.new
  doc[:s] = oid2
  doc[:n] = num
  col.save(:_id => doc)
end

query2 = BSON::OrderedHash.new
query2[:s] = oid
query2[:n] = 2

query3 = BSON::OrderedHash.new
query3[:s] = oid
query3[:n] = 3

pp col.find({
  :_id => {
    '$gte' => query2,
    '$lte' => query3,
  },
}).to_a

# Example output:
# [{"_id"=>{"s"=>BSON::ObjectId('4fc623684c114f24be000001'), "n"=>2}},
#  {"_id"=>{"s"=>BSON::ObjectId('4fc623684c114f24be000001'), "n"=>3}}]