Just added a bounty, here:
http://stackoverflow.com/questions/13204896/mongoid-save-fails-silently-without-embedded-in-relation

I have one document embedded in another in Mongoid.

    class A < B
      include Mongoid::Document
      embeds_one :shipping_address, class_name: 'Address'

I have, in my case, omitted the inverse relation:

    class Address
       # embedded_in :A

Why is it, that although the API works fine and completely as expected:

     address = A.address
     address.zip = 1234

     a.changed? #true

     address.save

     a.changed? #false

The document is not actually saved?

If i return the embedded_in statement, the save actually works fine.