Not sure if this is meant to be a breaking backwards change, but while upgrading to Mongoid 4 I can no longer use code that uses dynamic fields.

class User
  include Mongoid::Document
end

User.first.add_to_set(dynamic_field: 'foo') # => raises error

The error is caused by this line existing = send(field) || (attributes[field] ||= []).

Are atomic dynamic fields meant to no longer be supported?