All,

At present, I believe having a default_scope on a model will interfere with the uniqueness validation. If you add a default scope to the spec model located at spec/app/models/dictionary.rb such that it becomes:

class Dictionary
  include Mongoid::Document
  field :name, type: String
  field :publisher, type: String
  field :year, type: Integer
  field :published, type: Time
  has_many :words, validate: false
  default_scope where(published: 1999)
end

... then a large chunk of tests will fail. A further example is explained in this gist.
I do not have time at the moment to implement a patch, but will try to do so if someone else doesn't beat me to it. I bet it's a matter of making sure unscoped is used during the validation.

Thanks!