class Image
  include Mongoid::Document
  field :uploaded, type: Array
end

image = Image.create

image.atomically do
  image.add_to_set(:uploaded => '123')
  image.add_to_set(:uploaded => '456')
end

# MOPED: 127.0.0.1:27017 UPDATE       database=development collection=images selector={"_id"=>"51925dffc823202150000001"} update={"$addToSet"=>{"u"=>{"$each"=>["456"]}}} flags=[] (0.2313ms)

# Gemfile
rails (4.0.0.rc1)

# Gemfile.lock
GIT
  remote: git://github.com/mongoid/mongoid.git
  revision: df7f1fdd419c8613d53952992d9f5d233292db36