You may have to use in your model class or somewhere else the “context instance” in your code but when this code is used by a task, a test or data fixture, you get a “The default contect does not exist” error.
To avoid that, you can use the sfContext::hasInstance() fonction. See below

class Demande extends BaseDemande
{
   public function save(Doctrine_Connection $con = null)
   {
       if(sfContext::hasInstance())
       {
          $id_service = sfContext::getInstance()->getUser()->getAttribute('id_service', false, 'sfMatchlib/user');        
          $this->setIdService($id_service);
        }
        parent::save($con);
    }
}