在Hibernate中实现非关联实体间的ID引用与高效查询

import javax.persistence.*;
import java.time.LocalDateTime;

// ParentEntity.java
@Entity
@Table(name = "parent_entity")
public class ParentEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id; // Hibernate生成的ID
    private String name;

    // 构造函数、Getter和Setter
    public ParentEntity() {}
    public ParentEntity(String name) { this.name = name; }

    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; }

    @Override
    public String toString() {
        return "ParentEntity{id=" + id + ", name='" + name + "'}";
    }
}

// ChildLogEntity.java
@Entity
@Table(name = "child_log_entity")
public class ChildLogEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // 这是一个普通字段，用于引用ParentEntity的ID，没有ORM映射注解
    private Long parentId; 

    private String description;
    private LocalDateTime logTimestamp;

    // 构造函数、Getter和Setter
    public ChildLogEntity() {}
    public ChildLogEntity(Long parentId, String description) {
        this.parentId = parentId;
        this.description = description;
        this.logTimestamp = LocalDateTime.now();
    }

    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public Long getParentId() { return parentId; }
    public void setParentId(Long parentId) { this.parentId = parentId; }
    public String getDescription() { return description; }
    public void setDescription(String description) { this.description = description; }
    public LocalDateTime getLogTimestamp() { return logTimestamp; }
    public void setLogTimestamp(LocalDateTime logTimestamp) { this.logTimestamp = logTimestamp; }

    @Override
    public String toString() {
        return "ChildLogEntity{id=" + id + ", parentId=" + parentId + ", description='" + description + "', logTimestamp=" + logTimestamp + "}";
    }
}

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.TypedQuery;
import java.util.List;

public class QueryExample {

    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("my-persistence-unit"); // 替换为你的持久化单元名
        EntityManager em = emf.createEntityManager();

        try {
            em.getTransaction().begin();

            // 1. 创建并保存 ParentEntity，获取生成的ID
            ParentEntity parent1 = new ParentEntity("Parent A");
            em.persist(parent1);
            ParentEntity parent2 = new ParentEntity("Parent B");
            em.persist(parent2);

            // 确保ID已生成并刷新到数据库
            em.flush(); 

            // 2. 使用 ParentEntity 的 ID 创建并保存 ChildLogEntity
            ChildLogEntity log1_p1 = new ChildLogEntity(parent1.getId(), "Log entry for Parent A - action 1");
            ChildLogEntity log2_p1 = new ChildLogEntity(parent1.getId(), "Log entry for Parent A - action 2");
            ChildLogEntity log1_p2 = new ChildLogEntity(parent2.getId(), "Log entry for Parent B - action 1");
            em.persist(log1_p1);
            em.persist(log2_p1);
            em.persist(log1_p2);

            em.getTransaction().commit();

            // 3. 执行HQL查询，连接 ParentEntity 和 ChildLogEntity
            System.out.println("\n--- 查询所有 ParentEntity 及其关联的 ChildLogEntity ---");
            String hql = "SELECT p, cl " +
                         "FROM ParentEntity p " +
                         "JOIN ChildLogEntity cl ON p.id = cl.parentId " +
                         "WHERE cl.description LIKE :keyword " + // 示例筛选条件
                         "ORDER BY p.id, cl.logTimestamp";

            TypedQuery<Object[]> query = em.createQuery(hql, Object[].class);
            query.setParameter("keyword", "%Parent A%"); // 查找描述中包含 "Parent A" 的日志

            List<Object[]> results = query.getResultList();

            for (Object[] result : results) {
                ParentEntity parent = (ParentEntity) result[0];
                ChildLogEntity childLog = (ChildLogEntity) result[1];
                System.out.println("Parent: " + parent.getName() + " -> Log: " + childLog.getDescription());
            }

        } catch (Exception e) {
            if (em.getTransaction().isActive()) {
                em.getTransaction().rollback();
            }
            e.printStackTrace();
        } finally {
            em.close();
            emf.close();
        }
    }
}