Rails3 / Mongoid：多对多关系和删除操作

I have the following model :

我有以下型号：

class User
  include Mongoid::Document
  include Mongoid::Timestamps

  has_and_belongs_to_many :challenged, class_name: "Event", inverse_of: "challengers"

  def challenge!(event)
    self.challenged << event
    self.save!
  end

  def unchallenge!(event)
    self.challenged.where(_id: event.id).destroy_all
    self.save!
  end
end

class Event
  include Mongoid::Document
  include Mongoid::Timestamps

  has_and_belongs_to_many :challengers, class_name: "User", inverse_of: "challenged"
end
class