無法從 Spring Boot 應用程序中設置的 DBRef 中刪除

我的組對像有一組@DBRef 的用戶，每個用戶同樣有一組@DBRef 的組：

public class Group {
    @Id
    private String id;
    @Indexed(unique = true)
    private String name;
    @DBRef(lazy=true)
    private Set<User> users;

    //...
}

public class User {
    @Id
    private String id;
    @Indexed(unique = true)
    private String email;
    private String password;
    private String role;
    @DBRef(lazy = true)
    private Set<Group> groups;
    //...
}

當我刪除一個用戶時，我當然必須將他從他的組中刪除：

Set<Group> subscribedGroups = userRepository.findByEmail(email).getGroups();
for (Group g : subscribedGroups) {
    Set<User> users = g.getUsers();
    users.remove(user);
    g.setUsers(users); // not sure if this line is necessary but it doesnt work regardless
    groupRepository.save(g);
}

這不起作用。remove(user)由於某種原因返回 false。它應該可以工作；我打印了每個成員的 ID users，然後是user.getId()，然後是結果remove(user)：

List of users:
61abd6f1c81ab948c31641f2

User to delete: 61abd6f1c81ab948c31641f2
Result of removal: false