hash attributes are not saved to database

gistfile1.txt

class User 
        include Mongoid::Document 
        field :game_stats, :type=>Hash 
end 

console commands to recreate:
 ruby-1.9.2-p0 >   u = User.create! :game_stats => {} 
 => #<User _id: 4d9c9f038533c541ce000001, game_stats: {}> 
ruby-1.9.2-p0 >   u.game_stats['147'] = {} 
 => {} 
ruby-1.9.2-p0 >   u.game_stats['147']['playcount'] = 0 
 => 0 
ruby-1.9.2-p0 >   u.changes 
 => {"game_stats"=>[{}, {"147"=>{"playcount"=>0}}]} 
ruby-1.9.2-p0 > 
ruby-1.9.2-p0 >   u.save 
 => true 
ruby-1.9.2-p0 >   u.reload 
 => #<User _id: 4d9c9f038533c541ce000001, game_stats: 
{"147"=>{"playcount"=>0}}> 
ruby-1.9.2-p0 > 
ruby-1.9.2-p0 >   u.game_stats['147']['playcount'] += 1 
 => 1 
ruby-1.9.2-p0 >   u.game_stats 
 => {"147"=>{"playcount"=>1}} 
ruby-1.9.2-p0 > 
ruby-1.9.2-p0 >   u.changes 
 => {} 
ruby-1.9.2-p0 >   u.changed? 
 => false 
ruby-1.9.2-p0 >   u.save! 
 => true 
ruby-1.9.2-p0 >   u.reload 
#changes to playcount are lost 
 => #<User _id: 4d9c9f038533c541ce000001, game_stats: 
{"147"=>{"playcount"=>0}}>