Changing a hash field member attribute does not set the object to dirty state and does not save the changes to the database.
to recreate: https://gist.github.com/906508

k776 Edit: According to MONGOID-800, this may be a bug in the retrieving of the data, rather than the saving of it. In either case, both saving and loading needs to be checked as part of this ticket, with tests.

k776 Edit: A description of the problem and a work around at http://spin.atomicobject.com/2011/04/13/mongoid-hash-field-types-watch-out