With the upcoming changes to allow the _id field to be redefined in the standard field syntax we no longer need to support the composite key functionality. This is much better left to the application level anyways. For those using composite keys they will simply just need to change to the following syntax:
Defaults are now applied after all the attributes are set so this now should be a trivial change. This also allows for some cleanup of the hackish code around this area, and the nasty string inflections Mongoid has.