When setting duplicate_fields_exception to true, one can no longer use STI. https://github.com/mongoid/mongoid/blob/master/lib/mongoid/traversable.rb#L172 tries adding a _type field to both super- and subclass. The latter fails when using the aforementioned option as https://github.com/mongoid/mongoid/blob/master/lib/mongoid/fields/validators/macro.rb#L58 will be true.