class Feed
  include MongoMapper::Document
  many :stories
end

class Story
  include MongoMapper::EmbeddedDocument
  key :title,String
end

Feed = Feed.first
story = Story.new(:title => "Hello,world!") 
Feed.push(Feed.id,:stories => story.to_mongo) #works fine
Feed.set({:_id => Feed.id,"stories.title" => "Hello,world!"},"stories.$.title" => "Foo") #works fine

Feed.pull( ??? )

如何使用拉动原子地删除故事？