Spring JPA:在使用多对多关系时如何插入/更新具有唯一列的新项

@Entity
public class EntityA {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // other fields

    @ManyToMany
    @JoinTable(
        name = "join_table",
        joinColumns = @JoinColumn(name = "entity_a_id"),
        inverseJoinColumns = @JoinColumn(name = "entity_b_id", unique = true)
    )
    private List<EntityB> entityBs;

    // getters and setters
}

@Entity
public class EntityB {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // other fields

    @ManyToMany(mappedBy = "entityBs")
    private List<EntityA> entityAs;

    // getters and setters
}

EntityA entityA = new EntityA();
EntityB entityB = new EntityB();

// 设置其他字段的值

entityA.getEntityBs().add(entityB);
entityB.getEntityAs().add(entityA);

repositoryA.save(entityA);

if (!repositoryB.existsById(uniqueValue)) {
    // 执行插入/更新操作
}