Hibernate - List sequence number auto-generation without gaps and starting from zero

Question

I have UserObject which contains a list of SearchLog objects. This search log list size is defined to be at max 5.
Now whenever I add a search log to the UserObject in SearchLog list I want it to be added in the first position. The SearchLog object has a sequence number which I want to be automatically generated by hibernate. So if the searchLog list contains 3 elements on adding 4th searchLog in first position, the sequence number of previously persistent objects should automatically change.

While trying to save the UserObject with a new SearchLog object in the searchLog list, I get the exception that the sequence number of Search log should be specified (its not null field in DB). If I set the sequence number explicitly it works fine, but what I want is that hibernate should automatically handle this.Can this be done? Or is setting sequence number manually the only way.

Here's what I have been trying-

public class UserObject {
  private Integer id;
  private String firstName;
  private List<SearchLog> searchLogs = new ArrayList<SearchLog>();
}

public class SearchLog {
    private Integer id;
    private String searchQuery;
    private Integer sequenceNumber;
private Integer userObjectId;
}  

And the corresponding hbm files-
UserObject.hbm.xml

<hibernate-mapping package="com.xxx.admin.bo">
  <class name="UserObject " table="USER" dynamic-update="true" dynamic-insert="true" discriminator-value="AT">
    <id name="id" type="java.lang.Integer" column="ID" >
      <generator class="identity"/>
    </id>
    <property name="firstName" type="java.lang.String" column="FIRST_NAME" length="30"/>
    <list name="searchLogs" table="SEARCH_LOG" cascade="all-delete-orphan" lazy="true" inverse="true" >
      <key column="USER_OBJECT_ID" />
      <index column="SEQUENCE_NUMBER" />
      <one-to-many class="SearchLog" />    
    </list>
  </class>
</hibernate-mapping>

SearchLog.hbm.xml

<hibernate-mapping package="com.xxx.admin.bo">
  <class name="SearchLog" table="SEARCH_LOG" dynamic-update="true" dynamic-insert="true" discriminator-value="AT">
    <id name="id" type="java.lang.Integer" column="ID">
      <generator class="identity"/>
    </id>
    <property name="searchQuery" type="java.lang.String" column="SEARCH_QUERY" length="250"/>
    <property name="sequenceNumber" type="java.lang.Integer" column="SEQUENCE_NUMBER" not-null="true" />
    <property name="userObjectId" type="java.lang.Integer" column="USER_OBJECT_ID"/>
  </class>
</hibernate-mapping>

Code block of save method

userObject.getSearchLogs().add(0, searchLog);
getUserDao().save(userObject);

Answer 1

In SearchLog.hbm.xml use this instead Notice insert="false" update="false" and removed not-null attribute

<property name="sequenceNumber" type="java.lang.Integer" column="SEQUENCE_NUMBER" insert="false" update="false"/>