has_one vs. defining as a Key for Embedded documents for MongoMapper and MongoDB

Question

The Source code is

class RealTimeDetail
  include MongoMapper::EmbeddedDocument
  key :url, String
  key :method, String
end

class TargetFeed
  include MongoMapper::Document  

  key :name, String, :null => false
  key :feed_type, String, :null => false 
  has_one :real_time_detail

end

When I do target_feed.real_time_detail = RealTimeDetail.new(:url => "http://example.com", :method => "get")

I get errored out.

Instead i've changed the TargetFeed to

class TargetFeed
  include MongoMapper::Document  

  key :name, String, :null => false
  key :feed_type, String, :null => false 
  key :real_time_detail, RealTimeDetail

end

This works but was wondering if this is the best way to go about it.