How do I exclude fields from an embedded document in Mongoid?

I have a Post document that has embedded tags. Sometimes I display only the title of a post and its tags. In those cases, I use the following query in mongoid:

Post.only(:title).find(id)

I then send the results of the query as json to the client. Unfortunately, the tag's bson id makes the json much larger than I need it to be. How do I exclude the "_id" field from the query?

Here are my models:

class Post
  include Mongoid::Document
  field :title, :type =>  String
  field :body, :type =>  String
  field :tags, :type =>  Array
  embeds_many :tags
end
class Tag
  include Mongoid::Document  
  field :tag, :type =>  String
  field :type, :type =>  String
  embedded_in :post
end