JPA应用中在无外键约束下高效检查子记录以维护引用完整性

// 假设的父实体
@Entity
public class Parent {
    @Id
    private Long id;

    @OneToMany(mappedBy = "parent", fetch = FetchType.LAZY)
    private List<Child> children;

    @PreRemove
    public void checkChildrenBeforeRemove() {
        // 这种方式在子记录数量庞大时效率极低
        if (children != null && !children.isEmpty()) {
            throw new IllegalStateException("Cannot delete parent: associated children exist.");
        }
    }
    // ... 其他属性和方法
}

// Parent.java
import jakarta.persistence.*;

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

    // 假设Parent和Child通过parentId字段关联
    // 注意：这里不再需要@OneToMany，因为我们将通过应用层查询维护关系
    // private List<Child> children;

    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; }
}

// Child.java
import jakarta.persistence.*;

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

    // 关联父实体的ID
    private Long parentId;

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

// ChildRepository.java
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;
import java.util.Optional;

@Repository
public interface ChildRepository extends JpaRepository<Child, Long> {
    /**
     * 查找与给定parentId关联的第一个子记录。
     * Spring Data JPA会自动将其优化为类似 SELECT * FROM children WHERE parent_id = ? LIMIT 1 的SQL查询。
     *
     * @param parentId 父实体的ID
     * @return 如果存在子记录，则返回包含第一个子记录的Optional；否则返回空的Optional。
     */
    Optional<Child> findFirstByParentId(Long parentId);
}

// ParentHasChildrenException.java (自定义异常)
public class ParentHasChildrenException extends RuntimeException {
    public ParentHasChildrenException(String message) {
        super(message);
    }
}

// ParentDeletionListener.java
import org.springframework.stereotype.Component;
import org.springframework.beans.factory.annotation.Autowired;
import jakarta.persistence.PreRemove;

@Component // 标记为Spring组件，使其可被Spring管理和注入
public class ParentDeletionListener {

    // 使用静态字段和setter注入，以确保Spring管理的bean可以被JPA生命周期回调方法访问
    private static ChildRepository childRepository;

    @Autowired
    public void setChildRepository(ChildRepository childRepository) {
        ParentDeletionListener.childRepository = childRepository;
    }

    /**
     * 在Parent实体被删除前执行的逻辑。
     *
     * @param parent 正在被删除的Parent实体实例。
     */
    @PreRemove
    public void preRemoveParent(Parent parent) {
        // 确保仓库已被注入
        if (childRepository == null) {
            // 在生产环境中，这应该通过日志记录或更健壮的错误处理来处理
            System.err.println("Error: ChildRepository not initialized in ParentDeletionListener.");
            throw new IllegalStateException("Application configuration error: ChildRepository is null.");
        }

        // 高效检查是否存在子记录
        Optional<Child> firstChild = childRepository.findFirstByParentId(parent.getId());

        if (firstChild.isPresent()) {
            throw new ParentHasChildrenException("无法删除ID为 " + parent.getId() + " 的父实体，因为它存在关联的子记录。");
        }
    }
}

// Parent.java (更新后的Parent实体)
import jakarta.persistence.*;

@Entity
@Table(name = "parents")
@EntityListeners(ParentDeletionListener.class) // 关联监听器
public class Parent {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String 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; }
}