Setting a JPA timestamp column to be generated by the database?

Question

In my SQL Server 2000 database, I have a timestamp (in function not in data type) column of type DATETIME named lastTouched set to getdate()<

Answer 1

I have this working well using JPA2.0 and MySQL 5.5.10, for cases where I only care about the last time the row was modified. MySQL will create a timestamp on first insertion, and every time UPDATE is called on the row. (NOTE: this will be problematic if I cared whether or not the UPDATE actually made a change).

The "timestamp" column in this example is like a "last-touched" column.x`

The code below uses a separate column "version" for optimistic locking.

private long version;
private Date timeStamp

@Version
public long getVersion() {
    return version;
}

public void setVersion(long version) {
    this.version = version;
}

// columnDefinition could simply be = "TIMESTAMP", as the other settings are the MySQL default
@Column(name="timeStamp", columnDefinition="TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP")
@Temporal(TemporalType.TIMESTAMP)
public Date getTimeStamp() {
    return timeStamp;
}

public void setTimeStamp(Date timeStamp) {
    this.timeStamp = timeStamp;
}

(NOTE: @Version doesn't work on a MySQL "DATETIME" column, where the attribute type is "Date" in the Entity class. This was because Date was generating a value down to the millisecond, however MySQL was not storing the millisecond, so when it did a comparison between what was in the database, and the "attached" entity, it thought they had different version numbers)

From the MySQL manual regarding TIMESTAMP :

With neither DEFAULT nor ON UPDATE clauses, it is the same as DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP.