Question

Hi I have a domain object lets say Student and its Roll no as a primary key

here is the sample mapping for it.

    @Id
@Column(name = "Roll_NO", unique = true, nullable = false)
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "Roll_NO_SEQ")
@SequenceGenerator(name = "Roll_NO_SEQ", sequenceName = "Roll_NO_SEQ", allocationSize = 1)
public Long getRollNo() {
    return this.rollNo;
}

issue : lets say if a particular student was deleted from the database, and then re-admitted at the time of re-admission i want to retain the old roll no . so when i call session.save hibernate assigns a new roll No based on the Sequence specified rather then what i am setting through setRollNo() method. is there any way to achieve this in hibernate?

Answer 1

Given that you cannot change the legacy code, Ryan has the right idea. I had to do basically the same thing some time ago in a personal project. There were two parts: the easy part is to allow the effective setting of an otherwise-autonumber-ed column’s ID…and the other is to make the ID generator stop overwriting that value when you go to Save().

Here’s the code to the FlexibleIDGenerator I used:

public class FlexibleIDGenerator extends IdentityGenerator implements Configurable {
   public static final String DEFAULT = "default";

   private IdentifierGenerator assignedGenerator;
   private IdentifierGenerator defaultGenerator;

@SuppressWarnings("unchecked")
public Serializable generate(SessionImplementor session, Object object) throws HibernateException {
      //boolean useDefault = false;

      if (object instanceof OverridableIdentity) {
          if (((OverridableIdentity) object).isIDOverridden()) {
              try {
                  Class cl = object.getClass().getSuperclass();
                  Method[] methods = cl.getDeclaredMethods();

                  for (int i = 0; i < methods.length; i++) {
                      if (methods[i].getName().equalsIgnoreCase("setId")) {
                          methods[i].invoke(object, Integer.valueOf((((OverridableIdentity) object).getOverriddenID())));
                      }
                  }
              } catch (Exception ex) {
                  ex.printStackTrace();
              }
              return assignedGenerator.generate(session, object);
          } else {
              return defaultGenerator.generate(session, object);
          }
      } else {
          return defaultGenerator.generate(session, object);
      }
   }

   public void configure(Type type, Properties params, Dialect d) throws MappingException {
      assignedGenerator = IdentifierGeneratorFactory.create("assigned", type, params, d);
      defaultGenerator = IdentifierGeneratorFactory.create("increment", type, params, d);          
   }
}

To use that for a class, you update your Hibernate mapping file like this:

        <id
        name="Id"
        type="integer"
        column="id"
    >
        <generator class="com.mypackage.FlexibleIDGenerator"/>
    </id>

One other detail: I added a method to my base object called “GetOverriddenID()” to avoid confusion about whether I’m using the “normal” ID (in Update() calls) or the overridden ones.

Hope that helps.