JPA：父实体含两个子实体时，将子实体关联到单一父实体

你可以通过以下几种方式解决这个JPA关联冗余的问题：

方法一：使用单向OneToOne关联 #

直接移除ShipmentAddress中的反向关联属性，仅在Shipment中维护正向的一对一关系。这种方式最简单，适合不需要从ShipmentAddress反向查询所属Shipment的场景。

修改后的ShipmentAddress类：

@Entity
public class ShipmentAddress {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @Column(nullable = false)
    private String street1;
    private String street2;

    // 移除所有指向Shipment的属性，仅保留地址相关字段
}

Shipment类保持原有代码不变，所有关联操作从Shipment端发起即可。

方法二：通过继承+鉴别器区分地址类型 #

将ShipmentAddress抽象为父类，创建两个子类分别对应发货地址和收货地址，父类中仅保留一个指向Shipment的关联属性，子类通过鉴别器字段区分类型。

1. 抽象父类ShipmentAddress：

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "address_type", discriminatorType = DiscriminatorType.STRING)
public abstract class ShipmentAddress {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @Column(nullable = false)
    private String street1;
    private String street2;

    @OneToOne
    private Shipment shipment;
}

2. 发货地址子类FromAddress：

@Entity
@DiscriminatorValue("FROM")
public class FromAddress extends ShipmentAddress {
    // 无需额外字段，继承父类所有属性
}

3. 收货地址子类ToAddress：

@Entity
@DiscriminatorValue("TO")
public class ToAddress extends ShipmentAddress {
    // 无需额外字段，继承父类所有属性
}

4. 修改Shipment类关联子类：

@Entity
@Table(name = "shipment")
public class Shipment {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @ManyToOne
    private AppUser user;

    @OneToOne(cascade = CascadeType.ALL, mappedBy = "shipment")
    private FromAddress fromAddress;

    @OneToOne(cascade = CascadeType.ALL, mappedBy = "shipment")
    private ToAddress toAddress;
}

方法三：添加地址类型标记+过滤关联 #

在ShipmentAddress中添加一个枚举字段标记地址类型（发货/收货），使用@Where注解在Shipment中过滤出对应类型的地址，同时仅保留一个指向Shipment的关联属性。

1. 定义地址类型枚举：

public enum AddressType {
    FROM, TO
}

2. 修改ShipmentAddress类：

@Entity
public class ShipmentAddress {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @Column(nullable = false)
    private String street1;
    private String street2;
    @Column(nullable = false)
    @Enumerated(EnumType.STRING)
    private AddressType type;

    @ManyToOne
    private Shipment shipment;
}

3. 修改Shipment类添加过滤条件：

@Entity
@Table(name = "shipment")
public class Shipment {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @ManyToOne
    private AppUser user;

    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "from_address_id")
    @Where(clause = "type = 'FROM'")
    private ShipmentAddress fromAddress;

    @OneToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinColumn(name = "to_address_id")
    @Where(clause = "type = 'TO'")
    private ShipmentAddress toAddress;
}

内容的提问来源于stack exchange，提问作者Johnny boy