Hibernate / JPA在合并后获取数据库中具有默认值的列的值

Posted 2023-03-23

【中文标题】Hibernate / JPA在合并后获取数据库中具有默认值的列的值

【英文标题】：Hibernate/JPA get value of column with default value in database after merge

【发布时间】：2016-07-19 21:45:50

【问题描述】：

案例：我想在合并对象后获取映射到数据库端默认值列的字段值。

表格：

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
)

类：

@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： 对象被插入数据库，所有默认值都正确生成。

问题： 为什么第二个 println 打印 null，但第一个 println 打印插入数据库的正确 Id 值？

【参考方案1】：

在执行此行之前：

System.out.println(insertedObject.getEditedOn());

您必须通过调用“刷新”（同步）您的更新：entityManager.flush();

【参考方案2】：

经过更多研究，我发现如果我使用默认数据库值，则不可能将它们放在从合并返回的实体中。就那么简单。我更改了方法并使用了@PrePersist 和@PreUpdate 注释。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();
    

为什么正确的 id 在返回的实体中？因为其实是Hibernate生成的@GeneratedValue(strategy = GenerationType.IDENTITY)