my_app_money.rb

module MyApp
  class Money
    include Mongoid::Fields::Serializable

    def cast_on_read?; true; end

    def deserialize(object)
      return nil if object.blank?
      ::Money.new(object[:cents] || object["cents"], object[:currency] || object["currency"])
    end

    def serialize(object)
      return nil if object.blank?
      {:cents => object.cents, :currency => object.currency_as_string}
    end
  end
end

my_model.rb

class MyModel
  include Mongoid::Document
  
  field :price, type: MyApp::Money
 
end