public function update(Doctrine_Table $table, array $fields, array $identifier)
    {
        if (empty($fields)) {
            return false;
        }

        $set = array();
        foreach ($fields as $fieldName => $value) {
            if ($value instanceof Doctrine_Expression) {
                $set[] = $this->quoteIdentifier($table->getColumnName($fieldName)) . ' = ' . $value->getSql();
                unset($fields[$fieldName]);
            } else if (is_null($value)) {
                $set[] = $this->quoteIdentifier($table->getColumnName($fieldName)) . ' = NULL';
                unset($fields[$fieldName]);
            } else {
                $set[] = $this->quoteIdentifier($table->getColumnName($fieldName)) . ' = ?';
            }
        }

        $params = array_merge(array_values($fields), array_values($identifier));

        $sql  = 'UPDATE ' . $this->quoteIdentifier($table->getTableName())
              . ' SET ' . implode(', ', $set)
              . ' WHERE ' . implode(' = ? AND ', $this->quoteMultipleIdentifier($table->getIdentifierColumnNames()))
              . ' = ?';

        return $this->exec($sql, $params);
    }