Hibernate非映射关系实体ID引用与查询策略

@Entity
@Table(name = "parent")
public class Parent {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    // 其他字段...

    // 省略 getter/setter
}

@Entity
@Table(name = "child")
public class Child {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String description;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "parent_id") // 这是一个JPA管理的关联
    private Parent parent;

    // 其他字段...

    // 省略 getter/setter
}

@Entity
@Table(name = "child_log")
public class ChildLog {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // 引用Parent的ID，但没有JPA关联映射
    @Column(name = "parent_id")
    private Long parentId;

    // 引用Child的ID
    @Column(name = "child_id")
    private Long childId;

    private String action; // 例如：CREATED, UPDATED, DELETED
    private Timestamp recordTimestamp; // 记录时间

    // 其他字段，如变更前后的数据快照等...

    // 省略 getter/setter
}

@Service
public class DataService {

    @PersistenceContext
    private EntityManager entityManager;

    @Transactional
    public void createParentAndChildren(String parentName, List<String> childDescriptions) {
        // 1. 创建并持久化 Parent 实体
        Parent parent = new Parent();
        parent.setName(parentName);
        entityManager.persist(parent);
        // 此时 parent.getId() 已经被 Hibernate 填充

        Long generatedParentId = parent.getId(); // 获取生成的Parent ID

        // 2. 为每个 Child 实体创建并持久化
        for (String description : childDescriptions) {
            Child child = new Child();
            child.setDescription(description);
            child.setParent(parent); // JPA管理的关联
            entityManager.persist(child);

            // 3. 为 Child 实体创建 ChildLog 记录
            ChildLog childLog = new ChildLog();
            childLog.setParentId(generatedParentId); // 使用获取到的Parent ID
            childLog.setChildId(child.getId()); // 使用Child的ID
            childLog.setAction("CREATED");
            childLog.setRecordTimestamp(new Timestamp(System.currentTimeMillis()));
            entityManager.persist(childLog);
        }
    }
}

// 查询所有Parent及其关联的ChildLog记录
public List<Object[]> findParentsWithChildLogs() {
    String hql = "SELECT p, cl " +
                 "FROM Parent p " +
                 "JOIN ChildLog cl ON p.id = cl.parentId"; // 核心：非映射JOIN...ON
    return entityManager.createQuery(hql, Object[].class).getResultList();
}

// 查询特定Parent ID下的ChildLog记录
public List<ChildLog> findChildLogsByParentId(Long parentId) {
    String hql = "SELECT cl " +
                 "FROM ChildLog cl " +
                 "WHERE cl.parentId = :parentId";
    return entityManager.createQuery(hql, ChildLog.class)
                        .setParameter("parentId", parentId)
                        .getResultList();
}

// 查询Parent及其ChildLog，并进行过滤或聚合
// 例如：获取所有Parent，以及每个Parent有多少条ChildLog记录
public List<Object[]> countChildLogsPerParent() {
    String hql = "SELECT p.name, COUNT(cl.id) " +
                 "FROM Parent p " +
                 "JOIN ChildLog cl ON p.id = cl.parentId " +
                 "GROUP BY p.name";
    return entityManager.createQuery(hql, Object[].class).getResultList();
}