push unique in mongodb

push_unique.rb

require 'pp'
require 'rubygems'
require 'mongo_mapper'

MongoMapper.database = 'testing'

class Site
  include MongoMapper::Document
  key :domain, String
  key :authorizations, Array
  
  # only pushes user id if not already in array
  def add_user(user)
    collection.update({
        :_id => _id, 
        :authorizations => {'$ne' => user._id}
      }, '$push' => {'authorizations' => user._id})
  end
end

class User
  include MongoMapper::Document
  key :name, String
end

User.collection.remove
Site.collection.remove

steve = User.create(:name => 'Steve')
john = User.create(:name => 'John')

ol = Site.create(:domain => 'orderedlist.com',    :authorizations => [steve._id, john._id])
bh = Site.create(:domain => 'bethelhoops.com',    :authorizations => [steve._id])
rt = Site.create(:domain => 'railstips.org',      :authorizations => [john._id])
jn = Site.create(:domain => 'johnnunemaker.com',  :authorizations => [john._id])

rt.add_user(steve)
rt.add_user(steve)
rt.add_user(steve)

pp Site.find(rt._id) # only has two user ids even though steve added multiple times