hibernate

Reading the timestamp data from the hibernate envers revinfo table

A short blog post in case this helps anyone!

We’re using Hibernate Envers to keep an audit trail of data changes. I thought I’d have a look into the (Postgres) database to get some stats on usage – how many records of a certain type we’re creating by month for example.

The “data_aud” tables have a “rev” field which joins to the “revinfo” table, which looks like this:

             Table "public.revinfo"
  Column   |          Type          | Modifiers 
-----------+------------------------+-----------
 id        | integer                | not null
 version   | bigint                 | not null
 timestamp | bigint                 | not null
 user_name | character varying(255) | not null

I was surprised to see that the timestamp was of type bigint – the DDL was generated by Envers, not us!

Attempting to get realistic dates out of the field using Postgres functions such as 

to_timestamp(revinfo.timestamp)

failed dismally.

I found this post which gives a working answer for Oracle: SQL query to get a human readable display of the *number* REVINFO.timestamp ?

With a bit of tweaking, here is a useful Postgres view which I created to make future querying a bit easier: 

select id, version, user_name, timestamp,
  to_char(
    TO_DATE('01/01/1970 00:00:00','DD/MM/YYYY HH24:MI:SS')
    + (r.timestamp /1000/60/60/24 ||' day')::interval, 'YYYY/MM/DD'
  ) as rev_date, 
  to_char(
    TO_DATE('01/01/1970 00:00:00','DD/MM/YYYY HH24:MI:SS') 
    + (r.timestamp /1000/60/60/24 ||' day')::interval, 'YYYY/MM'
  ) as rev_month 
from revinfo r;
59

Custom Hibernate Sequence Generator for Id field

I have a table with a primary key in the format M001, M002 etc (lets not think about what happens after M999 for now). I’m using Hibernate Annotations, and I found a great way of generating the Primary Key value for new Records:

First I created a database sequence to use. Then I implemented org.hibernate.id.IdentifierGenerator;

public class StockCodeGenerator implements IdentifierGenerator {

    private static Logger log = Logger.getLogger(StockCodeGenerator.class);

    public Serializable generate(SessionImplementor session, Object object)
            throws HibernateException {

        String prefix = "M";
        Connection connection = session.connection();
        try {

            PreparedStatement ps = connection
                    .prepareStatement("SELECT nextval ('seq_stock_code') as nextval");

            ResultSet rs = ps.executeQuery();
            if (rs.next()) {
                int id = rs.getInt("nextval");
                String code = prefix + StringUtils.leftPad("" + id,3, '0');
                log.debug("Generated Stock Code: " + code);
                return code;
            }

        } catch (SQLException e) {
            log.error(e);
            throw new HibernateException(
                    "Unable to generate Stock Code Sequence");
        }
        return null;
    }
}

Then, in my entity class, I simply annotate the id field like this:

@Id
@GenericGenerator(name="seq_id", strategy="my.package.StockCodeGenerator")
@GeneratedValue(generator="seq_id")
@Column(name = "stock_code", unique = true, nullable = false, length = 20)
public String getStockCode() {
    return this.stockCode;
}

It works really well!

[Thanks to Jejomar Dimayuga for this post http://blog.dagitab.com/htsrv/trackback.php?tb_id=30 which I have modified slightly to use a sequence rather than a table]

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