I'd like to add a Date/DateTime/Timestamp field on an entity object, that will be automatically created when the entity is created/persisted and set to "now", never to be updated again.

Other use cases included fields that were always updated to contain the last modification date of the entity.

I used to achieve such requirements in the mysql schema.

What's the best way to do this in Play! / JPA?

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There is a code snippet that you can adapt to achieve what your want. Take a look:

// Timestampable.java

package models;

import java.util.Date;

import javax.persistence.Column;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;
import javax.persistence.PrePersist;
import javax.persistence.PreUpdate;
import javax.persistence.Version;

import play.db.ebean.Model;

@MappedSuperclass
public class Timestampable extends Model {

  @Id
  @GeneratedValue
  public Long id;

  @Column(name = "created_at")
  public Date createdAt;

  @Column(name = "updated_at")
  public Date updatedAt;

  @Version
  public int version;

  @Override
  public void save() {
    createdAt();
    super.save();
  }

 @PrePersist
  void createdAt() {
    this.createdAt = this.updatedAt = new Date();
  }

  @PreUpdate
  void updatedAt() {
    this.updatedAt = new Date();
  }

[출처] https://stackoverflow.com/questions/8202154/how-to-create-an-auto-generated-date-timestamp-field-in-a-play-jpa