Rails: Update parent object when saving child

In my app, a Conversation has many Messages. How to I update the updated_at attribute of a Conversation when a new Message in that Conversation is created/saved?

I'm aware of :touch => true, which does this, but it also updates Conversation when a Message is destroyed, which is not what I want.

Thanks.

Models

class Conversation < ActiveRecord::Base
  has_many :messages 
end

class Message < ActiveRecord::Base
  belongs_to :conversation
end