I think this was raised in the past (https://github.com/mongoid/mongoid/issues/647), but currently there's no way to distinguish between "nil" and "unset in db". This is particularly useful for sparse indices, which will consider "null" to be a value for index purposes (contrary to user intention.)

Not sure if this is possible, but it would be very nice if we could introduce a constant Mongoid::Undefined < NilClass which, if used would guarantee a field stays unset in the DB (not set to null)

It might be also nice to have Mongoid::UndefinedArray < Array and Mongoid::UndefinedHash which are handled as unset in DB whenever they are equal to an empty array / empty hash respectively in Ruby.

More broadly, you could make a module Mongoid::UnsetIfBlank which could be included into any primitive, which would unset the field in the DB if self.blank? is true

(Maybe this is better to raise with BSON driver?)