/**
  * Check collision
  *
  * @return string|null Store-like message
  */
 function checkCollisions()
 {
     $this->completeField("sejour_id");
     if (!$this->sejour_id) {
         return null;
     }
     $affectation = new CAffectation();
     $affectation->sejour_id = $this->sejour_id;
     $affectations = $affectation->loadMatchingList();
     unset($affectations[$this->_id]);
     foreach ($affectations as $_aff) {
         if ($this->collide($_aff)) {
             return "Placement déjà effectué";
         }
     }
     return null;
 }