解决JPA OneToOne共享主键级联保存冲突

// 父实体：Student
@Entity
@Table(name = "student")
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY) // ID自动生成
    @Column(name = "STUDENT_ID")
    private Long id;

    // 假设此处配置了CascadeType.ALL，且Address的ID应与Student的ID相同
    // 这里的映射方式在实际应用中需要更正，见下文“正确映射共享主键”
    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "STUDENT_ID") // 错误的映射方式，这表示Student拥有Address的FK
    private Address address;

    private String name;

    // Getters and Setters
}

// 子实体：Address
@Entity
@Table(name = "address")
public class Address {
    @Id
    @Column(name = "ADDRESS_ID") // 此ID应与Student的ID共享
    private Long id;

    @OneToOne
    private Student student; // 双向关联

    private String street;

    // Getters and Setters
}

// 父实体：Student
@Entity
@Table(name = "student")
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY) // ID由数据库自动生成
    @Column(name = "STUDENT_ID")
    private Long id;

    private String name;

    // mappedBy 指示 Address 实体拥有关系的管理权（外键在 Address 表中）
    // 初始不使用 CascadeType.ALL，以便在服务层手动控制持久化顺序
    @OneToOne(mappedBy = "student", cascade = {CascadeType.PERSIST, CascadeType.MERGE}, orphanRemoval = true)
    private Address address;

    // Getters and Setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public Address getAddress() { return address; }
    public void setAddress(Address address) {
        this.address = address;
        if (address != null) {
            address.setStudent(this); // 维护双向关系
        }
    }
}

// 子实体：Address
@Entity
@Table(name = "address")
public class Address {
    @Id // 此ID将由@MapsId注解从Student实体的主键派生
    @Column(name = "ADDRESS_ID") // 数据库中的主键列名，同时也是外键列
    private Long id;

    @OneToOne(fetch = FetchType.LAZY)
    @MapsId // 告诉JPA，Address的主键(id)也是其关联Student的外键
    @JoinColumn(name = "ADDRESS_ID", referencedColumnName = "STUDENT_ID") // 外键列的详细定义
    private Student student;

    private String street;
    private String city;

    // Getters and Setters
    public Long getId() { return id; }
    // 注意：当使用@MapsId时，通常不直接通过setter设置ID，而是由JPA管理
    // public void setId(Long id) { this.id = id; }
    public String getStreet() { return street; }
    public void setStreet(String street) { this.street = street; }
    public String getCity() { return city; }
    public void setCity(String city) { this.city = city; }
    public Student getStudent() { return student; }
    public void setStudent(Student student) {
        this.student = student;
        // 如果Student的ID已经生成，@MapsId会自动处理Address的ID
    }
}

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

@Service
public class StudentAddressService {

    @PersistenceContext
    private EntityManager entityManager;

    @Transactional
    public void saveStudentWithAddress(Student student, Address address) {
        // 1. 持久化父实体 (Student)
        // 此时，Student的ID可能尚未生成，仍为null或默认值
        entityManager.persist(student);

        // 2. 刷新EntityManager，强制将Student插入数据库并获取其生成的ID
        // 刷新后，student对象的id字段将被数据库生成的实际ID填充
        entityManager.flush();

        // 3. 设置子实体 (Address) 的ID，使其与父实体ID共享
        // @MapsId 会在persist时自动处理，但手动设置可以确保在flush后ID的同步
        address.setId(student.getId());

        // 4. 建立父子实体间的双向关联（如果尚未设置）
        // 确保Address知道其关联的Student，这对于@MapsId至关重要
        address.setStudent(student);
        // 如果Student实体也需要持有Address引用，确保已设置
        student.setAddress(address);

        // 5. 持久化子实体 (Address)
        // 此时Address的ID已设置，不会违反NOT NULL约束
        entityManager.persist(address);

        // 事务将在方法成功执行后自动提交
    }

    // 示例用法
    public void demoSave() {
        Student student = new Student();
        student.setName("张三");

        Address address = new Address();
        address.setStreet("大学路1号");
        address.setCity("北京");

        saveStudentWithAddress(student, address);
        System.out.println("学生ID: " + student.getId() + ", 地址ID: " + address.getId());
    }
}