JPA/Hibernate分离实体为何仅部分合并？关联删除无数据库变更

从EntityManager获取实体A后关闭会话，将A置为detached状态，删除A关联的B实体后调用merge方法，数据库未产生对应变更：重新打开EntityManager后，仅A的name字段变更被合并，B实体列表的变更完全不生效；无任何异常、错误或警告，Hibernate日志仅显示查询语句，没有针对b_a中间表的删除操作。

但如果在同一个EntityManager实例中完成查询、删除关联实体、合并操作的全流程，功能可正常生效。

主测试类 #

public class Main {

    public static void main(String[] args) {
        A a = getAByIdFromRepository(2);
        B b = a.getB().stream().filter(i -> i.getId() == 1).findFirst().orElse(null);
        a.remove(b);
        repositoryMerge(a);
    }

    public static A getAByIdFromRepository(int id) {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("persistence_unit");
        EntityManager em = factory.createEntityManager();
        A a = em.find(A.class, id);
        em.close();
        factory.close();
        return a;
    }

    public static void repositoryMerge(A a) {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("persistence_unit");
        EntityManager entityManager = factory.createEntityManager();
        entityManager.getTransaction().begin();
        entityManager.merge(a);
        entityManager.getTransaction().commit();
        entityManager.close();
        factory.close();
    }
}

实体类A #

@Entity
@Table(name = "a")
public class A implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "a_id")
    protected int id;
    @Column(name = "a_name")
    protected String name;
    @ManyToMany(mappedBy = "a", fetch = FetchType.EAGER)
    protected List<B> b;

   // getter、setter等省略

    public void remove(B b) {
        this.getB().remove(b);
        b.getA().remove(this);
    }

}

实体类B #

@Entity
@Table(name = "b")
public class B implements Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "b_id")
    protected int id;
    @Column(name = "b_name")
    protected String name;
    @JoinTable(
            name = "b_a",
            joinColumns = @JoinColumn(name = "b_id"),
            inverseJoinColumns = @JoinColumn(name = "a_id"))
    @ManyToMany(fetch = FetchType.EAGER)
    protected List<A> a;

  // getter、setter等省略

}

正常生效的代码示例 #

EntityManagerFactory factory = Persistence.createEntityManagerFactory("persistence_unit");
EntityManager entityManager = factory.createEntityManager();
A a = entityManager.find(A.class, 2);
B b = a.getB().stream().filter(i -> i.getId() == 1).findFirst().orElse(null);
a.remove(b);
entityManager.getTransaction().begin();
entityManager.merge(a);
entityManager.getTransaction().commit();
entityManager.close();
factory.close();

1. 关联关系维护端定位错误：A的@ManyToMany注解使用了mappedBy = "a"，说明A是关联关系的被动端（inverse端），B才是主动维护端（owning端）——中间表b_a的增删改操作只能通过维护端B的集合变更触发。
2. Detached实体的merge行为限制：当A处于detached状态时，调用merge(a)只会同步A自身的基本属性（比如name），不会处理其关联集合的变更——Hibernate认为被动端的集合变更不应该触发中间表操作，且此时B也是detached状态，merge不会自动处理游离实体的集合修改。
3. 同EntityManager实例的状态差异：在同一个EntityManager中，A和B都是managed状态，Hibernate会追踪所有实体的状态变更，即使修改了被动端A的集合，也会同步维护端B的状态，从而触发中间表的更新。

方案1：操作维护端B并merge #

修改合并逻辑，同时merge被修改的B实体，确保维护端的集合变更被Hibernate捕获：

public static void repositoryMerge(A a, B b) {
    EntityManagerFactory factory = Persistence.createEntityManagerFactory("persistence_unit");
    EntityManager entityManager = factory.createEntityManager();
    entityManager.getTransaction().begin();
    // merge A
    A managedA = entityManager.merge(a);
    // merge B（此时B的集合已移除A，维护端变更会触发中间表操作）
    entityManager.merge(b);
    entityManager.getTransaction().commit();
    entityManager.close();
    factory.close();
}

方案2：重新获取Managed状态的实体进行修改 #

避免操作detached实体，在合并前重新从EntityManager中加载A和B的managed实例，再执行删除关联的操作：

public static void fixAndMerge(int aId, int bId) {
    EntityManagerFactory factory = Persistence.createEntityManagerFactory("persistence_unit");
    EntityManager entityManager = factory.createEntityManager();
    entityManager.getTransaction().begin();
    
    // 获取managed状态的A和B
    A managedA = entityManager.find(A.class, aId);
    B managedB = entityManager.find(B.class, bId);
    
    // 删除关联
    managedA.remove(managedB);
    
    entityManager.getTransaction().commit();
    entityManager.close();
    factory.close();
}

方案3：调整关联关系的维护端（可选） #

如果业务场景允许，将A设为关联关系的维护端，这样修改A的集合就能直接触发中间表的操作：

// 修改实体A的注解，移除mappedBy，添加@JoinTable
@Entity
@Table(name = "a")
public class A implements Serializable {
    // ...其他属性
    @JoinTable(
            name = "b_a",
            joinColumns = @JoinColumn(name = "a_id"),
            inverseJoinColumns = @JoinColumn(name = "b_id"))
    @ManyToMany(fetch = FetchType.EAGER)
    protected List<B> b;
}

// 修改实体B的注解，添加mappedBy
@Entity
@Table(name = "b")
public class B implements Serializable {
    // ...其他属性
    @ManyToMany(mappedBy = "b", fetch = FetchType.EAGER)
    protected List<A> a;
}

内容的提问来源于stack exchange，提问作者randomuser333