@items.to_json(:user => current_user)

meet.rb

  attr_accessor :current_user
  
  def serializable_hash(options={})
    options = {} if options.nil?
    self.current_user = options[:user]
    options[:methods] ||= [ :participated_friends_ids ]
    super(options)
  end
  
  def participated_friends_ids(user = self.current_user)
    unless user.nil?
      # do some work
    else
      []
    end
  end

Magic for personal serializing to json for each user separately.