issue-155.rb

require 'mongoid'

Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db('testing')
end

class Publisher
  include Mongoid::Document
  field :stuff, :type => Hash
  collection.remove
end
  
publisher = Publisher.create :stuff => { :hello => "world" }
publisher.stuff[:hello] = "hola!"
publisher.save
p Publisher.first.stuff
__END__
> bundle exec ruby issue-155.rb
{"hello"=>"hola!"}