How to map timestamp column to JPA type?

Question

I'm using Play! 1.2.4 and PostgreSQL 9.1. I created a table with a created_at column, of type: timestamp without time zone. It has a default value of now().

The problem comes when I fetch data using my Entity class for this table. The createdAt field is always coming back null. Here is the field definition:

@Column(name="created_at", insertable=false)
public java.sql.Date createdAt;

All the other fields are populated correctly. I have tried changing the field type to a Date, Calendar, Timestamp, etc., and tried adding a @Timestamp annotation. But no combination has proved successful.

Thanks in advance for any help!

For reference, here is the DDL I used to create the table.

CREATE TABLE Users
(
  id serial     NOT NULL,
  username text NOT NULL,
  email text    NOT NULL,
  password_hash text NOT NULL DEFAULT 0,
  created_at timestamp without time zone DEFAULT now(),
  CONSTRAINT Users_pkey PRIMARY KEY (id),
  CONSTRAINT Users_username_key UNIQUE (username)
);

Update: I think the problem has to do with using JPA to insert a record. The database populates a default value of now() for created_at, but somehow Hibernate doesn't know about it when fetching that row.

If I query for a row that already existed, the createdAt field is populated correctly! OK... this narrows down the problem.

Answer 1

You need to detach and fetch entity from db. Insert statement does not contain created_at field so it's not managed by JPA at the time of creation. In repository it would look like this:

@Transactional
public Log save(Log log) {
    this.em.persist(log);
    this.em.detach(log);
    return this.em.find(Log.class, log.getId());
}

Answer 2

Try this:

@Column(name="create_at", insertable=false)
@Temporal(TemporalType.TIMESTAMP)
private Date createAt;

Answer 3

I didn't solve the problem exactly, but I worked around it.

Instead of having the database supply the default value of now(), I expressed it in JPA with @PrePersist:

@Column(name="created_at", nullable=false)
@Temporal(TemporalType.TIMESTAMP)
public Date createdAt;

@PrePersist
protected void onCreate() {
    createdAt = new Date();
}

That works fine! Inspiration taken from this answer. I'm still not sure why Hibernate didn't get updated with the default value that was applied in the database. It was stuck thinking the value was still null.