Java hibernate 一对一: which side should be the owner

Question

所以我从这些简单的例子中学习，有 2 个表，USERS 和 USER_DETAILS，足够简单，每个用户都有 user_details 并且它是一对一的关系。所以这个样本是这样的，

@Entity
@Table(name = "USERS")
public class User {
   @Id
   @GeneratedValue(strategy = GenerationType.IDENTITY)
   @Column(name = "USR_ID")
   private long id;

   @Column(name = "USERNAME", nullable = false, unique = true)
   private String username;

   @Column(name = "PASSWORD")
   private String password;

   @OneToOne(cascade = CascadeType.ALL, mappedBy = "user", fetch = FetchType.LAZY)
   private UserDetail userDetail;

   //Setter and getter methods
}

@Entity
@Table(name = "USER_DETAILS")
public class UserDetail {
   @Id
   @GeneratedValue(strategy = GenerationType.IDENTITY)
   @Column(name = "USR_DET_ID")
   private long id;

   @Column(name = "FIRST_NAME")
   private String firstName;

   @Column(name = "LAST_NAME")
   private String lastName;

   @Column(name = "EMAIL")
   private String email;

   @Column(name = "DBO")
   private LocalDate dob;

   @OneToOne(fetch=FetchType.LAZY)
   @JoinColumn(name = "USR_ID")
   private User user;

   //Setter and Getter methods
}

如果您查看mappedBy，它位于User 而不是UserDetails 中。

Q1: so USER is the owner, if it calls save(), 
USER_DETAILS table will be updated as well ?

Q2: same examples put mappedBy in the USER_DETAILS side,
why people want to do this ?
How to determine which side to put mappedBy ? 

感谢您的帮助!

Answer 1

Q2:相同的示例将mappedBy放在USER_DETAILS一侧，为什么人们要这样做？如何确定将mappedBy放在哪一边？

In a bidirectional relationship, each entity has a relationship field or property that refers to the other entity. Through the relationship field or property, an entity class’s code can access its related object. If an entity has a related field, the entity is said to “know” about its related object.

您的示例中存在双向一对一关系。 User 和 UserDetail 实体都有一个关系字段。在两个实体上指定的 @OneToOne 注释。

For one-to-one bidirectional relationships, the owning side corresponds to the side that contains the corresponding foreign key.

关系的所有者是 UserDetail 实体。所有者有 @JoinColumn 注释来指定外键 (USR_ID)。

关系的反面(User)具有mappedBy 属性。

Q1:所以 USER 是所有者，如果它调用 save()，USER_DETAILS 表也会更新吗？

在您的示例中，UserDetail 是所有者。因此保存过程:

User user = new User();  // Ignoring the constructor parameters...
UserDetail userDetail = new UserDetail();  

user.setUserDetail(userDetail);
userDetail.setUser(user);

userRepository.save(user);

您只需要保存父级。这也将拯救 child 。