protected function _build_update()
{
$query = array();
$query[] = "UPDATE {$this->_quote_identifier($this->_table_name)} SET";
$field_list = array();
foreach ($this->_dirty_fields as $key => $value) {
if (!array_key_exists($key, $this->_expr_fields)) {
$value = '?';
}
$field_list[] = "{$this->_quote_identifier($key)} = {$value}";
}
$query[] = join(", ", $field_list);
$this->_add_id_column_conditions($query);
return join(" ", $query);
}