https://github.com/mongoid/mongoid/issues/1206

I've done required changes for scoped mass assignment.
in lib/mongoid/attributes/processing.rb I inserted a line

attrs = (role == :default ? sanitize_for_mass_assignment(attrs) : sanitize_for_mass_assignment(attrs, role))

It is rather ugly, but it is the most simple way to support both ActiveModel 3.0 and 3.1.
Also, I'made changes to Document#initialize, write_attributes method and added a assign_attributes method according to documentation here: http://api.rubyonrails.org/classes/ActiveRecord/Base.html#method-i-assign_attributes

Also, you should remember, that my specs will fail with errors in ActiveModel 3.0 that doesn't support it.