Gorm 两表多对多关联时，append 失效

type UserTag struct {
    gorm.Model
    TagName  string       `gorm:"type:varchar(50);unique;comment:'标签名'"`
    Category string       `gorm:"type:varchar(50);default:null;comment:标签类别"`
    Describe string       `gorm:"type:varchar(100);comment:'标签描述'"`
    Users    []*UserInfos `gorm:"many2many:users_to_tags;"`
}

type User struct {
        Id  int
        PayedUser            int        `gorm:"type:int(11);default:0;comment:'付费用户标识'" json:"payed_user"`
    InvitationCode       string     `gorm:"type:varchar(20)"`
    InvitationNum        int        `gorm:"type:int(11);default:0"`
    Tags                 []*UserTag `gorm:"many2many:users_to_tags;"`
}


err = demoMysqlDB.Debug().Model(&UserInfos{Id: 5,}).Association("Tags").
         Append(&UserTag{
                       TagName: "测试"，
          })

当我想给用户添加标签是，该标签数据库已经在存在，添加不生效，关联表中用户id关联到了0

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