I'm not sure if this is by design or a bug, but if you have an object without changes, and do a save against it, all validations are run (and additional queries executed as necessary to validate relations), but no insert is then performed.

/mongoid/persistence/operations/update.rb

has this:

        def persist
          prepare do
            unless updates.empty?
              collection.find(selector).update(positionally(selector, updates))
              ...

Why not do

def persist
   return if updates.empty?
   prepare do
    ...

I imagine that the consequence of this would be that save on an invalid object without changes would return true, which is potentially breaking, but since the end result is the same (no update to the object in the DB), it seems like save is equally valid as a return result.