mongoid_hash_issue.rb

# HELLO?
class Foo
  include Mongoid::Document
  field :stats, :type => Hash
end

problem.rb

f = Foo.create(:stats => {"bar" => {}})

f.stats["bar"]["baz"] = "qux"
# => <Foo _id: 4da45df77654aa7729000004, stats: {"bar"=>{"baz"=>"qux"}}>
f.save!
# => true

# looks good
f.stats["bar"]
# => {"baz"=>"qux"}

# oh noes, what happened?
f.reload.stats["bar"]
# => {}

workaround.rb

class Object
  def deep_copy
    Marshal.load(Marshal.dump(self))
  end
end
f = Foo.create(:stats => {"bar" => {}})
stats_copy = f.stats.deep_copy
stats_copy["bar"]["baz"] = "qux"

f.stats = stats_copy
f
# => #<Foo _id: 4da45f987654aa7729000005, stats: {"bar"=>{"baz"=>"qux"}}>
f.save!
# => true

# looks good
f.stats["bar"]
# => {"baz"=>"qux"}

# victory...
f.reload.stats["bar"]
# => {"baz"=>"qux"}