如何在Spring JPA中实现一个实体关联多个不同实体？

需要实现投诉功能：User可以投诉多种不同实体（Question、Answer、Comment或其他User），数据库schema设计为单表complains，其中user_id关联发起投诉的用户（一对多），entity_id关联被投诉的实体（多对一）。

尝试用泛型类Complain实现，但抛出异常：

org.hibernate.AnnotationException: 
Property com.*.*.Entities.Complain.entity_id has an unbound type and no explicit target entity.
Resolve this Generic usage issue or set an explicit target attribute (eg @OneToMany(target=)
or use an explicit @Type...

原有代码如下：

Complain.java（泛型版本） #

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@ToString
@Entity
@Table(name = "complains")
public class Complain<T extends BaseComplain> {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "user_id", nullable = false)
    private User user_id;

    @ManyToOne
    @JoinColumn(name = "entity_id", nullable = false)
    private T entity_id;

    @Column
    @Temporal(TemporalType.TIMESTAMP)
    private Date created_on;

}

User.java #

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@ToString
@Entity
@Table(name = "users")
public class User extends BaseComplain {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    @OneToMany(mappedBy = "user_id", orphanRemoval = true, fetch = FetchType.LAZY)
    @ToString.Exclude
    private Set<Complain<BaseComplain>> author_complains;

    @OneToMany(mappedBy = "entity_id", orphanRemoval = true, fetch = FetchType.LAZY)
    @ToString.Exclude
    private Set<Complain<User>> complains;

    // 其他内容...
}

Question.java（其他实体实现类似） #

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@ToString
@Entity
@Table(name = "questions")
public class Question extends BaseComplain {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @OneToMany(mappedBy = "entity_id", orphanRemoval = true, fetch = FetchType.LAZY)
    @ToString.Exclude
    @JsonManagedReference
    private Set<Complain<Question>> complains;
    
    // 其他内容...
}

问题：能否仅使用JPA基础特性实现该逻辑？已考虑过@MappedSuperclass，需要具体实现方案。

核心问题分析 #

JPA规范不支持泛型实体类，Hibernate无法在运行时解析未绑定的泛型参数T，因此抛出类型未绑定的异常。需要用多态关联+映射超类的方式替代泛型实现。

步骤1：重构BaseComplain为@MappedSuperclass #

将所有可被投诉的实体的公共主键字段抽离到映射超类中：

@MappedSuperclass
public abstract class BaseComplain {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // 公共getter/setter
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
}

步骤2：修改可被投诉的实体 #

让User、Question等实体继承BaseComplain，移除重复的id字段：

User.java

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@ToString
@Entity
@Table(name = "users")
public class User extends BaseComplain {
    // 移除原有id字段，继承自BaseComplain

    @OneToMany(mappedBy = "user", orphanRemoval = true, fetch = FetchType.LAZY)
    @ToString.Exclude
    private Set<Complain> authorComplains;

    @OneToMany(mappedBy = "targetEntity", orphanRemoval = true, fetch = FetchType.LAZY)
    @ToString.Exclude
    private Set<Complain> complains;

    // 其他原有字段...
}

Question.java

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@ToString
@Entity
@Table(name = "questions")
public class Question extends BaseComplain {
    // 移除原有id字段，继承自BaseComplain

    @OneToMany(mappedBy = "targetEntity", orphanRemoval = true, fetch = FetchType.LAZY)
    @ToString.Exclude
    @JsonManagedReference
    private Set<Complain> complains;

    // 其他原有字段...
}

步骤3：重构Complain实体（去掉泛型） #

添加实体类型字段解决不同实体id冲突问题，直接关联BaseComplain：

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@ToString
@Entity
@Table(name = "complains")
public class Complain {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "user_id", nullable = false)
    private User user;

    @ManyToOne
    @JoinColumn(name = "entity_id", nullable = false)
    private BaseComplain targetEntity;

    // 用枚举存储实体类型，比字符串更安全
    @Enumerated(EnumType.STRING)
    @Column(name = "entity_type", nullable = false)
    private EntityType entityType;

    @Column
    @Temporal(TemporalType.TIMESTAMP)
    private Date createdOn;

    // 辅助方法：设置目标实体时自动填充类型
    public void setTargetEntity(BaseComplain targetEntity) {
        this.targetEntity = targetEntity;
        if (targetEntity instanceof User) {
            this.entityType = EntityType.USER;
        } else if (targetEntity instanceof Question) {
            this.entityType = EntityType.QUESTION;
        } else if (targetEntity instanceof Answer) {
            this.entityType = EntityType.ANSWER;
        } else if (targetEntity instanceof Comment) {
            this.entityType = EntityType.COMMENT;
        }
    }
}

步骤4：定义EntityType枚举 #

public enum EntityType {
    USER, QUESTION, ANSWER, COMMENT
}

查询示例 #

查询某个用户收到的所有投诉：

List<Complain> userComplains = entityManager.createQuery(
        "SELECT c FROM Complain c WHERE c.targetEntity.id = :userId AND c.entityType = :userType",
        Complain.class
)
.setParameter("userId", userId)
.setParameter("userType", EntityType.USER)
.getResultList();

内容的提问来源于stack exchange，提问作者p0var