Spring Boot Data JPA双向关联外键为空问题求助

我正在开发一个超级英雄与反派的Spring Boot演示项目，使用JPA实现实体关联。抽象类Character被Superhero和Villain继承，Team与Character是一对多关系（一个团队包含多个角色，一个角色仅属于一个团队）。初始化数据后，数据库中superheroes表的team_id字段始终为空，尝试过添加referencedColumnName、调整级联类型、查询后再设置属性等方法都没解决。

实体代码 #

抽象Character类 #

@Data
@NoArgsConstructor
@Entity
@Table(name = "characters")
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "type")
@JsonSubTypes({
        @JsonSubTypes.Type(value = Superhero.class, name = "superhero"),
        @JsonSubTypes.Type(value = Villain.class, name = "villain")})
public abstract class Character {
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    private Integer id;
    private String firstName;
    private String lastName;
    @Transient
    private String fullName;
    @Convert(converter = GenderConverter.class)
    private Gender gender;
    @Column(nullable = false)
    private String alias;
    @Convert(converter = StatusConverter.class)
    private Status status;
    @ManyToOne
    @JoinColumn(name = "team_id")
    private Team team;
    private String originStory;
    @Convert(converter = UniverseConverter.class)
    private Universe universe;

    public Character(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public Character(String firstName, String lastName, String alias) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.alias = alias;
    }

    public String getFullName() {
        return firstName + " " + lastName;
    }
}

Team类 #

@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
@Entity
@Table(name = "teams")
public class Team {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String name;
    @OneToMany(mappedBy = "team", cascade = CascadeType.ALL)
    private List<Character> characters;
    @Convert(converter = StatusConverter.class)
    private Status status;
}

初始化数据代码 #

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    /** Disable before running Tests! */
    @Bean
    public CommandLineRunner commandLineRunner(CharacterRepository characterRepository,
                                               TeamRepository teamRepository) {
        return args -> {
            // Initilaizing
            Superhero ironMan = new Superhero("Tony", "Stark", "Iron Man");
            Superhero thor = new Superhero("Thor", "Odinson", "God of Thunder");
            Team avengers = Team.builder().name("Avengers").status(DISBANDED).build();

            // Setting attributes
            ironMan.setGender(MALE); thor.setGender(MALE);
            ironMan.setUniverse(MCU); thor.setUniverse(MCU);
            ironMan.setStatus(DECEASED); thor.setStatus(ALIVE);

            // Saving to repositories
            characterRepository.saveAll(List.of(ironMan, thor));
            teamRepository.save(avengers);

            // Trying to assign foreign keys
            avengers.setCharacters(List.of(ironMan, thor));
            ironMan.setTeam(avengers);
            thor.setTeam(avengers);
        };
    }
}

数据库查询结果 #

demo2=# SELECT * FROM teams;
 id |   name   |  status
----+----------+-----------
  1 | Avengers | Disbanded
(1 row)


demo2=# SELECT * FROM superheroes;
 id | team_id |     alias      | first_name | gender | last_name | origin_story |  status  | universe
----+---------+----------------+------------+--------+-----------+--------------+----------+----------
  1 |         | Iron Man       | Tony       | Male   | Stark     |              | Deceased | MCU
  2 |         | God of Thunder | Thor       | Male   | Odinson   |              | Alive    | MCU
(2 rows)

问题原因 #

1. 关联维护端逻辑：@OneToMany(mappedBy = "team")明确了Character是关联的维护端（持有外键team_id），只有修改维护端的team属性并保存，才会触发数据库外键更新；Team的characters集合是被维护端，仅设置该集合不会同步到数据库。
2. 未保存关联变更：当前代码先保存了Character和Team，之后才设置关联关系，但没有重新保存Character实体，JPA无法感知后续的关联修改。

解决方案 #

方案一：先建立关联再级联保存 #

利用Team的CascadeType.ALL配置，先建立双向关联，再保存Team即可自动保存关联的Character，同时填充外键：

@Bean
public CommandLineRunner commandLineRunner(CharacterRepository characterRepository,
                                           TeamRepository teamRepository) {
    return args -> {
        // 初始化实体
        Superhero ironMan = new Superhero("Tony", "Stark", "Iron Man");
        Superhero thor = new Superhero("Thor", "Odinson", "God of Thunder");
        Team avengers = Team.builder().name("Avengers").status(DISBANDED).build();

        // 建立双向关联
        ironMan.setTeam(avengers);
        thor.setTeam(avengers);
        avengers.setCharacters(List.of(ironMan, thor));

        // 设置其他属性
        ironMan.setGender(MALE); thor.setGender(MALE);
        ironMan.setUniverse(MCU); thor.setUniverse(MCU);
        ironMan.setStatus(DECEASED); thor.setStatus(ALIVE);

        // 级联保存Team及关联的Character
        teamRepository.save(avengers);
    };
}

方案二：先保存Team，再关联并保存Character #

先保存Team获取主键ID，再给Character设置关联属性并保存：

@Bean
public CommandLineRunner commandLineRunner(CharacterRepository characterRepository,
                                           TeamRepository teamRepository) {
    return args -> {
        // 先保存Team获取ID
        Team avengers = Team.builder().name("Avengers").status(DISBANDED).build();
        teamRepository.save(avengers);

        // 初始化角色并设置关联
        Superhero ironMan = new Superhero("Tony", "Stark", "Iron Man");
        Superhero thor = new Superhero("Thor", "Odinson", "God of Thunder");
        
        ironMan.setTeam(avengers);
        thor.setTeam(avengers);
        avengers.setCharacters(List.of(ironMan, thor));

        // 设置其他属性
        ironMan.setGender(MALE); thor.setGender(MALE);
        ironMan.setUniverse(MCU); thor.setUniverse(MCU);
        ironMan.setStatus(DECEASED); thor.setStatus(ALIVE);

        // 保存角色，触发外键更新
        characterRepository.saveAll(List.of(ironMan, thor));
    };
}

方案三：原有顺序下重新保存关联实体 #

如果坚持原有操作顺序，需在设置关联后重新保存Character：

// Trying to assign foreign keys
avengers.setCharacters(List.of(ironMan, thor));
ironMan.setTeam(avengers);
thor.setTeam(avengers);

// 重新保存角色，同步外键到数据库
characterRepository.saveAll(List.of(ironMan, thor));

内容的提问来源于stack exchange，提问作者saba morchilashvili