I did this with Mongoid

mongoid_initializer.rb

# Monkeypatching BSON::Object
module BSON
  class ObjectId
    # These aliases override default JSON representation of an ObjectId
    # Spits a string id '540f51de6d61632c74020000', instead of
    # { "$oid": '540f51de6d61632c74020000' }
    alias :to_json :to_s
    alias :as_json :to_s
  end
end

# Monkeypatching Mongoid::Document
module Mongoid::Document

  # Overrides ActiveModel::Serialization#serializable_hash for Mongoid Docs
  # If needed, please refer to the original method documentation.
  # Returns serialized_hash of any Mongoid::Document with following conditions:
  #
  #  - For a root document (not embedded) the `:_id` field is replaced for
  #    `:id` field
  #  - For an embedded document :_id field is completely excluded
  #
  # It gracefully keeps any other :only, :except, and :methods
  # options originally passed in the `options` hash
  def serializable_hash(options = nil)
    options ||= {}
    if self.embedded?
      options[:except] = Array(options[:except]) << :_id
      if options.has_key?(:methods)
        options[:methods] = Array(options[:methods])
        options[:methods].delete(:id)
      end
    else
      options[:methods] = Array(options[:methods]) << :id
      options[:except] = Array(options[:except]) << :_id
    end
    super options
  end
end