Problem as described in the gist.

In particular, if during the life of the embedding object, an instance of the embeded class is assigned that was previously assigned, the embedding is immediately cleared in the database, with the ruby-side object being frozen.

Tested on:

• ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]
• mongoid 3.0.5