mm_index_information.rb

require 'mongo_mapper'
MongoMapper.database = 'testing'

class User
  include MongoMapper::Document
end

class Bookmark
  include MongoMapper::Document
  belongs_to :user
  ensure_index :user_id
end

class Tag
  include MongoMapper::Document
  belongs_to :user
  belongs_to :bookmark
  ensure_index :user_id
  ensure_index :bookmark_id
  ensure_index :user_id => Mongo::ASCENDING, :bookmark_id => Mongo::ASCENDING
end

require 'pp'
pp User.collection.index_information
pp Bookmark.collection.index_information
pp Tag.collection.index_information