Group By / Filters with UltraSphinx

Hi,

I use last version of UltraSphinx but filters dont work...

In my model :
is_indexed :fields => ['content', 'titre', 'created_at', {:field =>
'lang', :facet => true, :as => 'lang'}],
                :delta => true
or
is_indexed :fields => ['content', 'titre', 'created_at', 'lang'],
                :delta => true

In my controller :
@search = Ultrasphinx::Search.new(:query =>
get_query_by_subject(@subject.id),
                                        :sort_mode => 'descending',
                                        :sort_by => 'created_at',
                                        :weights => {'titre' => 5.0,
'content' => 2.0},
                                        :page => params[:page] || 1,
                                        :filters => {"lang" => "fr"})

Result :
UsageError : field "lang" is invalid

:conditions => {"lang" => "fr"} doesnt work too.

I want to filter my results (lang, country, date, etc...) and group by
title. Ultrasphinx doc doesnt speak about that. Just query work ^^

And I dont really understand what are "facets". Can you help me
please ?

Thank you,
Florian