The Mongoid::Errors::DocumentNotFound error has a attribute reader for params (the parameters used to unsuccessfully find the document(s)) but that attribute is not being set.

To fix just need to add the @params = params line to document_not_found.rb:23:

 ruby
if !unmatched && !params.is_a?(Hash)
  raise ArgumentError, 'please also supply the unmatched ids'
end
@params = params

It would be nice if it stored the klass in an attribute too so that from the exception I can generate my own error which contains info about the error and generate a 404 page.