$factures = $this->Commande->find('all', array(

                    'conditions' => array(
                        'Commande.prestataire_id' => $this->request->data['prestataire_id']

                    ),
             'group' => array('Commande.date'),
                 'order' => array('Commande.date' => 'DESC'),
                 'fields' => array(' Commande.date'),
                ));