合并后，Hibernate / JPA获取数据库中具有默认值的列的值

Franek 发表于 Dev

坦率

案例：合并对象后，我想获取映射到数据库端具有默认值的列的字段的值。

桌子：

CREATE TABLE IF NOT EXISTS `db`.`OBJECT` (
  `OBJECT_ID` INT NOT NULL AUTO_INCREMENT,
  `NAME` VARCHAR(64) NULL,
  `CREATED_ON` DATETIME NULL DEFAULT CURRENT_TIMESTAMP,
  `EDITED_ON` DATETIME NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
)

班级：

@Entity
@Table(name="OBJECT")
public class SomeObject {
    private int id;
    private String name;
    private Timestamp createdOn;
    private Timestamp editedOn;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID")
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @Column(name = "NAME")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Column(name = "CREATED_ON", insertable = false, updatable = false)
    public Timestamp getCreatedOn() {
        return createdOn;
    }

    public void setCreatedOn(Timestamp createdOn) {
        this.createdOn = createdOn;
    }

    @Column(name = "EDITED_ON", insertable = false, updatable = false)
    public Timestamp getEditedOn() {
        return editedOn;
    }

    public void setEditedOn(Timestamp editedOn) {
        this.editedOn = editedOn;
    }
}

DAO:

@Transactional
public class ObjectDAO {
    @PersistenceContext
    private EntityManager entityManager;

    public someObject merge(someObject someObject) {
        return entityManager.merge(someObject);
    }
}

我想做的事：

SomeObject someObject = new SomeObject();
someObject.setName("name");
SomeObject insertedObject = objectDAO.merge(someObject);
System.out.println(insertedObject.getId());
System.out.println(insertedObject.getEditedOn());

结果：对象已插入数据库，所有默认值均已正确生成。

问题：为什么第二个println打印空值，而第一个println打印插入到数据库中的Id的正确值？

坦率

经过更多研究，我发现如果我使用默认数据库值，则不可能在合并返回的实体中使用它们。就那么简单。我更改方法，并使用@PrePersist和@PreUpdate注释。这应该是这样的：

@Entity
@Table(name="OBJECT")
public class SomeObject {
    private int id;
    private String name;
    private Timestamp createdOn;
    private Timestamp editedOn;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID")
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    @Column(name = "NAME")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Column(name = "CREATED_ON")
    public Timestamp getCreatedOn() {
        return createdOn;
    }

    public void setCreatedOn(Timestamp createdOn) {
        this.createdOn = createdOn;
    }

    @Column(name = "EDITED_ON")
    public Timestamp getEditedOn() {
        return editedOn;
    }

    public void setEditedOn(Timestamp editedOn) {
        this.editedOn = editedOn;
    }

    @PrePersist
    public void setDates() {
        createdOn = LocalDateTime.now();
        editedOn = LocalDateTime.now();
    }

    @PreUpdate
    public void setEditedOn() {
        editedOn = LocalDateTime.now();
    }
}