code: protected function _authenticateCreateSelect()
      {
          $dbSelect = clone $this->getDbSelect();
          $dbSelect->from($this->_tableName)
              ->where("{$this->_identityColumn} = ?", $this->_identity)
              ->limit(1);
  
          return $dbSelect;
      }
docs: Creates a Zend_Db_Select object that is completely configured to be
  queried against the database.
  
  @return Zend_Db_Select
  @see http://framework.zend.com/manual/1.12/en/zend.db.select.html Zend_Db_Select