protected function delete(array $criteria = array())
    {
        $conditions = array();
        
        foreach ($criteria as $field => $value) {
            $conditions[] = "$field = ?";
        }
        
        $sql = "DELETE FROM $this->_table WHERE " . implode(' AND ', $conditions);
        
        $pdos = $this->_pdo->prepare($sql);
        
        $pdos->execute(array_values($criteria));
        
        return $pdos->statement()->rowCount();
    }