/**
  * Init field definition object 
  *
  * @access private
  * @param string import id
  * 
  */
 private function initFieldObject($a_id, $a_type)
 {
     switch ($this->getMode()) {
         case self::MODE_INSERT:
         case self::MODE_INSERT_VALIDATION:
             $this->current_field = ilAdvancedMDFieldDefinition::getInstanceByTypeString($a_type);
             $this->fields[] = $this->current_field;
             return true;
         default:
             // ??? nonsense
             $this->current_field = ilAdvancedMDRecord::_getInstanceByFieldId($this->extractFieldId($a_id));
             return true;
             break;
     }
 }