如何在Gorm中附加多对多关系，而不颠倒已存在的关联行？

type User struct {
    gorm.Model
    Name   string
    Roles  []Role `gorm:"many2many:user_roles;ForeignKey:UserID;AssociationForeignKey:RoleID"`
}

type Role struct {
    gorm.Model
    Name  string
    Users []User `gorm:"many2many:user_roles;ForeignKey:RoleID;AssociationForeignKey:UserID"`
}

// 创建用户和角色
user1 := User{Name: "user1"}
user2 := User{Name: "user2"}
role1 := Role{Name: "role1"}
role2 := Role{Name: "role2"}

db.Create(&user1)
db.Create(&user2)
db.Create(&role1)
db.Create(&role2)

// 关联用户和角色
db.Model(&user1).Association("Roles").Append(&role1)
db.Model(&user1).Association("Roles").Append(&role2)
db.Model(&user2).Association("Roles").Append(&role1)

// 获取用户所拥有的角色
var roles []Role
db.Model(&user1).Association("Roles").Find(&roles)