CREATE TABLE `arc_test` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `data` datetime DEFAULT NULL,
  `text` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`,`data`)
) ENGINE=InnoDB;

<hibernate-reverse-engineering>  
    <table name="arc_test" catalog="db">
        <primary-key>
            <generator class="native"/>
        </primary-key>       
    </table> 
</hibernate-reverse-engineering> 

public class ArcTest implements java.io.Serializable
{
    private ArcTestId   id;
    private String      text;
    .....
    .....

public class ArcTestId implements java.io.Serializable
{
    private Integer id;
    private Date    data;
        .....
        .....

   public class ArcTest implements java.io.Serializable
    {
        private Integer id;
        private Date    data;
        private String  text;
        .....
        .....